Free HESI A2 Practice Test: 105 Questions With Explanations

Answer: D. 4 1/6 hours

Give both fractions the same denominator: 1/2 = 3/6 and 2/3 = 4/6. Add the whole numbers (1 + 2 = 3) and the fractions (3/6 + 4/6 = 7/6). Since 7/6 = 1 1/6, the total is 3 + 1 1/6 = 4 1/6 hours.

Why the other choices are wrong:

• A. 3 1/6 hours — Keeps the extra 1/6 but drops the whole hour hiding inside 7/6.
• B. 3 3/5 hours — Adds straight across, (1 + 2)/(2 + 3), which isn’t how fractions add.
• C. 4 hours — Comes from rewriting 1/2 as 2/6 instead of 3/6.

Source: worked solution shown above.

Answer: A. 2 1/2 yards

The fractional part 1/4 is smaller than 3/4, so regroup one whole: 5 1/4 = 4 5/4. Then 4 5/4 − 2 3/4 = 2 2/4 = 2 1/2 yards. Check by adding back: 2 1/2 + 2 3/4 = 5 1/4.

Why the other choices are wrong:

• B. 3 yards — Subtracts only the whole numbers (5 − 2) and ignores the fractions.
• C. 3 1/2 yards — Subtracts the fractions in the wrong order (3/4 − 1/4) to avoid regrouping.
• D. 8 yards — Adds the two lengths instead of subtracting.

Source: worked solution shown above.

Answer: 8 servings

Divide the total by the size of one serving. Dividing by a fraction means multiplying by its reciprocal: 6 ÷ 3/4 = 6 × 4/3 = 24/3 = 8 servings. Check: 8 × 3/4 = 6 cups.

Common mistake: Multiplying 6 × 3/4 gives 4 1/2. That’s three-quarters of the pitcher, not the number of servings.

Source: worked solution shown above.

Answer: B. 0.84

Multiply as if there were no decimal points: 24 × 35 = 840. The two factors have 3 decimal places in total (one in 2.4, two in 0.35), so the product needs 3: 0.840, or 0.84. Estimate to check: 0.35 is about 1/3, and a third of 2.4 is 0.8.

Why the other choices are wrong:

• A. 0.084 — Uses four decimal places instead of three.
• C. 8.4 — Uses only one decimal place.
• D. 840 — Ignores the decimal points entirely.

Source: worked solution shown above.

Answer: C. 37.5%

Divide 3 by 8 to get the decimal 0.375. To write a decimal as a percent, multiply by 100: 0.375 × 100 = 37.5%.

Why the other choices are wrong:

• A. 0.375% — Stops at the decimal and adds a percent sign without multiplying by 100.
• B. 3.8% — Writes the digits 3 and 8 side by side; that isn’t a conversion.
• D. 38% — Rounds 37.5 to 38, so it isn’t exactly equal to 3/8.

Source: worked solution shown above.

Answer: 36 masks

Change 15% to a decimal (0.15) and multiply: 0.15 × 240 = 36. A quick mental check: 10% of 240 is 24 and 5% is half of that, 12, so 15% is 24 + 12 = 36.

Common mistake: Dividing 240 by 15 gives 16, which answers a different question.

Source: worked solution shown above.

Answer: B. 15%

Percent change = amount of change ÷ original amount × 100. The price fell by $80 − $68 = $12, and 12 ÷ 80 = 0.15, or 15%.

Why the other choices are wrong:

• A. 12% — Treats the $12 drop as if it were already a percent.
• C. 17.6% — Divides by the new price ($68) instead of the original price ($80).
• D. 85% — Is the new price as a percent of the old one (68 ÷ 80), not the decrease.

Source: worked solution shown above.

Answer: B. 24

Cross-multiply: 5 × x = 3 × 40, so 5x = 120 and x = 24. Check: 24/40 simplifies to 3/5.

Why the other choices are wrong:

• A. 8 — Divides 40 by 5 but forgets to multiply by 3.
• C. 66.7 — Sets up the cross-multiplication upside down (3x = 5 × 40), which gives 200 ÷ 3.
• D. 120 — Stops after cross-multiplying and forgets to divide by 5.

Source: worked solution shown above.

Answer: 3 mL

Set up a proportion with the units lined up: 250 mg / 5 mL = 150 mg / x mL. Cross-multiply: 250x = 750, so x = 3 mL. Check: 150 mg is 3/5 of 250 mg, and 3/5 of 5 mL is 3 mL. (This is arithmetic practice, not dosing guidance.)

Common mistake: An answer of about 8.3 comes from flipping the proportion (250 × 5 ÷ 150).

Source: worked solution shown above.

Answer: C. 12

1 fluid ounce = 2 tablespoons, and 1 tablespoon = 3 teaspoons. So 2 fl oz = 4 tbsp, and 4 × 3 = 12 teaspoons.

Why the other choices are wrong:

• A. 4 — Stops at tablespoons (2 fl oz = 4 tbsp).
• B. 6 — Treats each fluid ounce as a tablespoon (2 × 3).
• D. 16 — Multiplies by 8, mixing up “8 fluid ounces in a cup” with teaspoons.

Source: NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement — p. C-26: 1 tablespoon = 3 teaspoons = 1/2 fluid ounce (exact)

Answer: 360 mL

Convert cups to fluid ounces, then fluid ounces to milliliters: 1 1/2 × 8 = 12 fl oz, and 12 × 30 = 360 mL. The 30 mL figure is a rounded kitchen value; NIST lists a U.S. fluid ounce as about 29.573 mL, so always use the factor a problem gives you.

Common mistake: An answer of 240 mL converts only the first cup and forgets the extra half.

Source: NIST, Metric Kitchen: Cooking Measurement Equivalencies — Cooking equivalents table: rounded 30 mL per fluid ounce; given exercise factors determine 360 mL.

Related explanations: NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement — Printed C-25 (U.S. fluid ounce 29.573 mL, rounded); C-27 (kg/lb).

Answer: 70 kg

A kilogram is heavier than a pound, so the number of kilograms will be smaller than the number of pounds. Divide: 154 ÷ 2.2 = 70 kg. (NIST lists 1 kg as about 2.205 lb; 2.2 is the rounded factor the question tells you to use.)

Common mistake: Multiplying (154 × 2.2 = 338.8) converts in the wrong direction.

Source: NIST, Metric Kitchen: Cooking Measurement Equivalencies — Equivalencies table: 1 kg ≈ 2.2 pounds; NIST HB 44 App. C, p. C-27

Related explanations: NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement — Printed C-25 (U.S. fluid ounce 29.573 mL, rounded); C-27 (kg/lb).

Answer: D. 40 °C

Use °C = (°F − 32) ÷ 1.8. Subtract first: 104 − 32 = 72. Then divide: 72 ÷ 1.8 = 40 °C.

Why the other choices are wrong:

• A. 219.2 °C — Uses the Celsius-to-Fahrenheit formula (× 1.8 + 32), which converts in the wrong direction.
• B. 72 °C — Subtracts 32 but forgets to divide by 1.8.
• C. 57.8 °C — Divides by 1.8 but forgets to subtract 32 first.

Source: NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement — p. C-20, Conversion Equations for Units of Temperature

Answer: C. 1945

A 24-hour clock keeps counting after noon instead of starting over at 1. For p.m. times from 1:00 to 11:59, add 12 to the hour: 7 + 12 = 19, so 7:45 p.m. is 1945.

Why the other choices are wrong:

• A. 0745 — Is 7:45 a.m.
• B. 1745 — Adds 10 to the hour instead of 12.
• D. 2045 — Adds 13 to the hour instead of 12.

Source: worked solution shown above.

Answer: B. XCIV

Build it one place value at a time. 90 is XC (10 written before 100 means 100 − 10). 4 is IV (1 before 5 means 5 − 1). Together, 94 = XCIV.

Why the other choices are wrong:

• A. IVC — I is only placed before V or X to subtract, so IVC isn’t a standard numeral.
• C. XCVI — XCVI is 96, because VI means 6, not 4.
• D. LXXXXIV — Standard numerals don’t repeat X four times; 90 is written XC.

Source: Mathwords, Roman Numerals — Subtractive pairs IV, IX, XL, XC, CD, CM

Answer: C. After finding that many visitors waited in the wrong line, the clinic changed how it directs people to check-in.

Paragraph 1 describes the problem (41 visitors waited in the wrong line), paragraphs 2 and 3 explain what the coordinator noticed and what changed, and paragraph 4 states the goal. Only this choice covers the whole passage.

Why the other choices are wrong:

• A. Riverside Community Clinic moved its laboratory and pharmacy to make check-in easier. — The passage describes changes to signs and directions, not a move of the laboratory or pharmacy.
• B. Volunteers are always more effective than signs at guiding clinic visitors. — The passage never compares volunteers with signs in general, and “always” goes far beyond it.
• D. Most late appointments at the clinic are caused by visitors pushing strollers. — Paragraph 2 says people with strollers or wheelchairs could miss the sign, not that they caused most late appointments.

Evidence: Paragraphs 1–4 of the passage.

Answer: D. Weekday mornings

Paragraph 3 places the greeter near the entrance “each weekday morning, when the clinic is busiest.”

Why the other choices are wrong:

• A. Every afternoon — Paragraph 4 says the clinic will decide in June whether to add afternoons.
• B. Only during the two-week review — The review is how the clinic measured the problem, not the greeter’s schedule.
• C. Whenever the laboratory is busy — The passage never ties the greeter’s hours to the laboratory.

Evidence: Paragraphs 3–4 of the passage.

Answer: A. So people pushing strollers or using wheelchairs would be more likely to see them

Paragraph 2 says the old sign hung above eye level, where people pushing strollers or using wheelchairs could easily miss it. Lower signs answer that exact problem.

Why the other choices are wrong:

• B. Because there was no room left on the upper walls — The passage says nothing about wall space.
• C. Because the green floor line was hard to see — Nothing suggests the floor line was hard to see; it was one of the fixes.
• D. Because the signs in two languages needed more space — The passage never discusses sign size.

Evidence: Paragraphs 2–3 of the passage.

Answer: B. recorded

Staff “logged 41 visitors” during a two-week review, which means they kept a written count of them. Recorded fits the sentence.

Why the other choices are wrong:

• A. delayed — The visitors were late, but the staff didn’t delay them.
• C. cut down — That’s a different meaning of log (cutting trees), which doesn’t fit here.
• D. ignored — Ignoring visitors is the opposite of counting them.

Evidence: Paragraph 1 of the passage.

Answer: D. To explain a problem at a clinic and the steps taken to fix it

The passage informs: it describes a problem, what staff observed, the three changes, and the plan to check the results.

Why the other choices are wrong:

• A. To persuade readers to volunteer at a community clinic — There’s no call to volunteer.
• B. To criticize the clinic’s front-desk staff — The front-desk staff tracked the problem; nothing criticizes them.
• C. To compare two clinics’ check-in systems — Only one clinic is described.

Evidence: Whole passage of the passage.

Answer: C. Volunteers completed more practice questions per hour in the silent rooms.

Paragraph 2 reports exactly this. It’s the only choice that stays within what the logs actually measured.

Why the other choices are wrong:

• A. Silent rooms help every student learn more. — The logs covered eight volunteers, not every student, and they counted practice questions, not learning.
• B. Open tables are too noisy for studying. — Noise wasn’t measured; volunteers noted fewer interruptions in the silent rooms.
• D. Students stayed much longer in the silent rooms. — Paragraph 2 says sessions lasted about as long in both places.

Evidence: Paragraph 2 of the passage.

Answer: D. “Everyone should study in the silent rooms.”

Marcus Lee’s statement in paragraph 3 tells people what they should do. It’s a judgment about the results, not something the logs recorded.

Why the other choices are wrong:

• A. Sessions in the silent rooms lasted about as long as sessions at the open tables. — This is a finding from the logs.
• B. Volunteers completed more practice questions per hour in the silent rooms. — This is also a finding from the logs.
• C. Eight volunteers kept a simple log for three weeks. — This describes how the information was collected.

Evidence: Paragraphs 1–3 of the passage.

Answer: B. The volunteers chose where to study, so students who already felt focused may have picked the silent rooms.

Paragraph 3 gives two concerns: the volunteers chose their own locations, and eight volunteers is a small group. This choice restates the first one.

Why the other choices are wrong:

• A. The volunteers forgot to record how long they studied. — Paragraph 1 says the volunteers recorded how long they stayed.
• C. The silent rooms were closed for part of the three weeks. — The passage never mentions closures.
• D. The practice questions were easier in the silent rooms. — The passage never compares question difficulty.

Evidence: Paragraph 3 of the passage.

Answer: A. It presents two different views of what the results mean.

Marcus Lee thinks the results are clear; Professor Shah explains why they might not be. The paragraph sets those two views side by side.

Why the other choices are wrong:

• B. It explains how the volunteers collected their information. — That’s paragraph 1.
• C. It describes the club’s final decision. — That’s paragraph 4.
• D. It lists the practice questions the volunteers used. — No questions are listed anywhere in the passage.

Evidence: Paragraph 3 of the passage.

Answer: B. A small study found more practice questions per hour in the silent rooms, but because of its limits, the club shared the findings without making a rule and planned a larger follow-up.

This choice keeps the main finding, the study’s limits, and the club’s actual decision in paragraph 4.

Why the other choices are wrong:

• A. A study proved that silent rooms are best, so the club now requires them. — Paragraph 4 says the club chose not to make a rule, and nothing was “proved.”
• C. The study club found no difference between the two study locations. — The logs did show a difference in questions per hour.
• D. The adviser rejected the study because the volunteers were not honest. — Professor Shah questioned the study’s design, not anyone’s honesty.

Evidence: Whole passage of the passage.

Answer: A. A new patient care technician learns how a careful, structured handoff helps nurses pass on information accurately.

Every paragraph follows Jordan’s first look at handoff: what she expected, what she saw, the pattern she noticed, and the reason Ms. Levin gives for repeating information back.

Why the other choices are wrong:

• B. Night-shift nurses give longer reports than day-shift nurses. — Only one night nurse’s reports appear, and they aren’t compared with day-shift reports.
• C. Nursing handoffs delay breakfast service. — Paragraph 3 mentions breakfast trays only to show that nobody rushed.
• D. Patients are often dizzy early in the morning. — One patient was dizzy; the passage makes no general claim.

Evidence: Whole passage of the passage.

Answer: C. She wants to remember why repeating information back matters.

Jordan asked why Ms. Levin repeated the request, heard the reason, and kept the sentence in her badge holder. That points to wanting to remember the lesson.

Why the other choices are wrong:

• A. She plans to report Ms. Levin to the charge nurse. — Nothing suggests Jordan disapproves of Ms. Levin.
• B. She needs a label for her badge holder. — The badge holder is where she keeps the card, not what the card is for.
• D. She disagrees with Ms. Levin and plans to argue later. — Nothing suggests disagreement; she asked a question and kept the answer.

Evidence: Paragraph 4 of the passage.

Answer: D. The charge nurse asks Jordan to sit in on the morning handoff.

Paragraph 1 opens with the charge nurse’s request. The other events follow in paragraphs 2, 3 and 4.

Why the other choices are wrong:

• A. Ms. Levin repeats the night nurse’s request back to him. — That happens in paragraph 2.
• B. Jordan notices that every report follows the same order. — That happens in paragraph 3.
• C. Jordan writes a sentence on an index card. — That happens last, in paragraph 4.

Evidence: Paragraphs 1–4 of the passage.

Answer: A. clear and exact

Mr. Okafor’s request says exactly what to do and when: help the patient the first time she walks this morning. Specific means clear and exact.

Why the other choices are wrong:

• B. unusual — Nothing suggests the request was strange.
• C. optional — He’s asking the day nurse to do it; it isn’t presented as optional.
• D. medical — The word describes how precise the request is, not its subject.

Evidence: Paragraph 2 of the passage.

Answer: C. thorough

He talks for almost three minutes and covers who the patient is, why she was admitted, how her night went, and what the next nurse should do.

Why the other choices are wrong:

• A. careless — Covering four parts of the report carefully is the opposite of careless.
• B. hurried — Paragraph 3 says nobody rushed.
• D. humorous — Nothing in the report is meant to be funny.

Evidence: Paragraphs 2–3 of the passage.

Answer: A. harmful

Adverse means harmful or unfavorable, so “no adverse reactions” means no harmful ones. Don’t confuse it with averse, which means opposed to something.

Why the other choices are wrong:

• B. expected — Expected describes whether a reaction was predicted, not whether it was harmful.
• C. mild — A mild reaction can still be adverse; the words aren’t synonyms.
• D. delayed — Delayed describes timing.

Source: Merriam-Webster, adverse

Answer: C. make worse

Exacerbate means to make something worse, such as a symptom or a problem.

Why the other choices are wrong:

• A. relieve — Relieve is close to the opposite.
• B. hide — Hiding pain doesn’t change how bad it is.
• D. measure — Measuring pain describes it; it doesn’t worsen it.

Source: Merriam-Webster, exacerbate

Answer: B. sluggish and lacking energy

Lethargic means sluggish, drowsy, and low on energy, which fits someone worn out by three night shifts.

Why the other choices are wrong:

• A. anxious — Anxious means worried or uneasy.
• C. confused — Confused is about unclear thinking, not low energy.
• D. irritable — Irritable means easily annoyed.

Source: Merriam-Webster, lethargic

Answer: D. affecting both sides

Bilateral means on or affecting both sides. The prefix bi- means two, and lateral refers to a side.

Why the other choices are wrong:

• A. sudden — Sudden describes timing.
• B. affecting one side — Affecting one side is unilateral, the opposite.
• C. mild — Mild describes severity.

Source: Merriam-Webster, bilateral

Answer: A. able to be felt by touch

Palpable means able to be felt by touching. To palpate is to examine by feeling with the hands.

Why the other choices are wrong:

• B. painful — A palpable lump may or may not hurt; the word says nothing about pain.
• C. growing quickly — The word says nothing about growth.
• D. visible only on an X-ray — Something palpable can be felt directly, without imaging.

Source: Merriam-Webster, palpable

Related explanations: Merriam-Webster, palpate — Entry definitions relevant to the word in context; not automatically generated example sentences.

Answer: C. long-lasting

Chronic means continuing for a long time or recurring often. Its common opposite in health settings is acute, which describes something sudden and short-lived.

Why the other choices are wrong:

• A. sudden and brief — That describes acute, the opposite.
• B. severe — Chronic pain can be mild or severe; the word is about duration.
• D. rare — Rare describes how often something occurs in a population.

Source: Merriam-Webster, chronic

Related explanations: Merriam-Webster, acute — Adjective and medical senses: rapid onset and relatively short duration.

Answer: A. brief but complete

Concise means expressing a lot in few words. A three-sentence summary has to be brief and still cover the key points.

Why the other choices are wrong:

• B. highly detailed — Highly detailed is close to the opposite.
• C. formal — Formal describes tone, not length.
• D. careless — Concise writing is careful, not careless.

Source: Merriam-Webster, concise

Answer: C. careful to protect privacy

Discreet means careful about what you say or do, especially to protect privacy.

Why the other choices are wrong:

• A. separate and distinct — That’s discrete, a different word that sounds the same.
• B. loud and clear — Loud and clear is the opposite of discreet.
• D. quick — Speed isn’t part of the meaning.

Source: Merriam-Webster, discreet

Answer: B. about to happen

Impending means about to happen. The clouds and wind are warning signs of a storm that hasn’t arrived yet.

Why the other choices are wrong:

• A. recently ended — That’s the opposite time frame.
• C. imaginary — The storm is expected, not imagined.
• D. mild — Impending describes timing, not strength.

Source: Merriam-Webster, impending

Answer: D. become worse

Deteriorate means to become worse in condition or quality over time.

Why the other choices are wrong:

• A. improve — Improve is the opposite.
• B. stay the same — Continuing to deteriorate means changing for the worse, not staying the same.
• C. grow larger — The word is about condition, not size.

Source: Merriam-Webster, deteriorate

Answer: B. preventive

Prophylactic means intended to prevent a disease or problem.

Why the other choices are wrong:

• A. emergency — An emergency visit responds to a problem that already exists.
• C. painful — The word says nothing about pain.
• D. cosmetic — Cosmetic means done for appearance.

Source: Merriam-Webster, prophylactic

Answer: C. slow

Brady- means slow, so bradycardia describes a slow heart rate. Its opposite is tachy- (fast), as in tachycardia.

Why the other choices are wrong:

• A. fast — Fast is tachy-.
• B. irregular — Irregular isn’t the meaning of brady-.
• D. strong — Strong isn’t the meaning of brady-.

Source: MedlinePlus (NLM), Appendix A: Word Parts and What They Mean — Tables for body parts, problems/procedures, speed, amount, and position; brady-, -itis, hypo-, -ectomy and distractor affixes.

Answer: A. inflammation

-itis means inflammation, so tendinitis is inflammation of a tendon. Inflammation can have many causes, so -itis doesn’t automatically mean infection.

Why the other choices are wrong:

• B. surgical removal — Surgical removal is -ectomy.
• C. study of — Study of is -logy.
• D. enlargement — Enlargement is -megaly.

Source: MedlinePlus (NLM), Appendix A: Word Parts and What They Mean — Tables for body parts, problems/procedures, speed, amount, and position; brady-, -itis, hypo-, -ectomy and distractor affixes.

Answer: B. below normal

Hypo- means below or under normal, so hypothermia is a body temperature below normal. Its opposite is hyper- (above normal).

Why the other choices are wrong:

• A. above normal — Above normal is hyper-.
• C. around — Around is peri- or circum-.
• D. after — After is post-.

Source: MedlinePlus (NLM), Appendix A: Word Parts and What They Mean — Tables for body parts, problems/procedures, speed, amount, and position; brady-, -itis, hypo-, -ectomy and distractor affixes.

Answer: D. surgical removal

-ectomy means surgical removal, so an appendectomy removes the appendix. Don’t mix it up with -otomy (cutting into) or -scopy (looking inside with a scope).

Why the other choices are wrong:

• A. cutting into — Cutting into is -otomy.
• B. viewing with a scope — Viewing with a scope is -scopy.
• C. inflammation — Inflammation is -itis.

Source: MedlinePlus (NLM), Appendix A: Word Parts and What They Mean — Tables for body parts, problems/procedures, speed, amount, and position; brady-, -itis, hypo-, -ectomy and distractor affixes.

Answer: B. is

The subject is box, which is singular. Of gloves is a prepositional phrase describing the box, so it doesn’t control the verb. A singular subject takes is.

Why the other choices are wrong:

• A. are — Agrees with gloves, the wrong word.
• C. were — Were does not agree with the singular subject box in this sentence.
• D. be — Be isn’t a complete verb form in this sentence.

Source: Purdue OWL, Subject/Verb Agreement — Rules 3 and 5: nearest or/nor subject; intervening prepositional phrases.

Answer: A. is

With neither…nor, the verb agrees with the subject closer to it. The closer subject is the doctor, which is singular, so use is.

Why the other choices are wrong:

• B. are — Agrees with nurses, the farther subject.
• C. were — Were does not agree with the closer singular subject doctor in this sentence.
• D. have been — Plural helping verb.

Source: Purdue OWL, Subject/Verb Agreement — Rules 3 and 5: nearest or/nor subject; intervening prepositional phrases.

Answer: C. I

The pronoun is part of the subject, so it takes the subject form, I. Test it alone: “I volunteered” works; “me volunteered” doesn’t.

Why the other choices are wrong:

• A. me — Me is an object pronoun.
• B. her — Her is also an object form here.
• D. my — My is a possessive form that needs a following noun here.

Source: Purdue OWL, Pronoun Case — Subject, object and possessive table; compound subject/object checks.

Answer: D. me

The pronoun follows the preposition to, so it takes the object form, me. Test it alone: “gave them to me.”

Why the other choices are wrong:

• A. I — I is a subject pronoun.
• B. we — We is a subject pronoun, and it’s plural.
• C. my — My is a possessive form that needs a following noun here.

Source: Purdue OWL, Pronoun Case — Subject, object and possessive table; compound subject/object checks.

Answer: B. drunk

After had, use the past participle. Drink is irregular: drink (present), drank (past), drunk (past participle).

Why the other choices are wrong:

• A. drank — Drank is the simple past, not the past participle.
• C. drinked — Drinked isn’t a standard form.
• D. drinking — “Had drinking” isn’t a correct verb phrase.

Source: Merriam-Webster, drink

Answer: C. When I arrived, the lecture had already started.

Both actions are in the past. The past perfect (had started) marks the earlier one, and the simple past (arrived) marks the later one.

Why the other choices are wrong:

• A. When I arrived, the lecture has already started. — Has started is present perfect, which doesn’t match a past arrival.
• B. When I arrive, the lecture had already started. — Arrive is present tense.
• D. When I arrived, the lecture will already start. — Will start points to the future.

Source: Purdue OWL, Verb Tense Consistency — Perfect tenses and relations between past events.

Answer: A. well

The blank describes how she answered, which is an action (a verb). Words that describe verbs are adverbs, and the adverb here is well. Good is an adjective, so it describes nouns.

Why the other choices are wrong:

• B. good — Good is an adjective.
• C. best — “Answered very best” isn’t standard; best is a superlative.
• D. more good — More good isn’t standard; the comparative is better.

Source: Purdue OWL, Adjectives and Adverbs — The Basic Rules: Adverbs; adjectives versus adverbs.

Related explanations: Merriam-Webster, well — Adverb senses and Good vs. Well Usage Guide.

Answer: D. The clinic opens at eight; the lab opens at nine.

The original joins two complete sentences with only a comma, which is a comma splice. A semicolon can join two closely related independent clauses.

Why the other choices are wrong:

• A. The clinic opens at eight the lab opens at nine. — Removing the comma creates a run-on (fused) sentence.
• B. The clinic opens at eight, the lab, opens at nine. — Still a comma splice, plus an extra comma between subject and verb.
• C. The clinic opens, at eight the lab opens at nine. — Puts a comma inside the first clause and leaves no valid boundary between the two independent clauses.

Source: Purdue OWL, Independent and Dependent Clauses — Comma Splices and Fused Sentences; semicolon repair.

Answer: B. The patient waited near the window.

It has a subject (patient), a verb (waited), and a complete thought. The other choices are fragments.

Why the other choices are wrong:

• A. Because the waiting room was crowded. — Starts with because, which makes it a dependent clause.
• C. After checking the schedule twice. — A phrase with no subject.
• D. Which the nurse had cleaned earlier. — Here, which introduces a dependent relative clause that needs a main clause.

Source: Purdue OWL, Sentence Fragments — Dependent-clause fragments, including relative which clauses.

Answer: C. the students’ backpacks

For a plural noun that already ends in s, add only an apostrophe: students’.

Why the other choices are wrong:

• A. the student’s backpacks — Student’s means the backpacks belong to one student.
• B. the students backpacks — Without an apostrophe, nothing shows ownership.
• D. the students’s backpacks — A plural ending in s doesn’t take an extra ’s.

Source: Purdue OWL, Apostrophe Introduction — Plural nouns already ending in s; possessive pronouns; contractions.

Answer: A. its

Its (no apostrophe) is the possessive form, like his or hers. It’s always means “it is” or “it has.”

Why the other choices are wrong:

• B. it’s — “It is visiting hours” makes no sense here.
• C. its’ — Its’ is never correct.
• D. it is — Same problem as it’s.

Source: Purdue OWL, Apostrophe Introduction — Plural nouns already ending in s; possessive pronouns; contractions.

Answer: B. affect

Affect is usually the verb meaning “to influence.” Effect is usually a noun meaning a result. As a verb, effect means “to bring about,” which doesn’t fit here.

Why the other choices are wrong:

• A. effect — As a verb, effect means “to bring about,” so the sentence would say lack of sleep creates the ability.
• C. effects — A noun or a verb form that doesn’t fit after can.
• D. affection — Affection is a noun meaning fondness.

Source: Merriam-Webster, affect

Answer: C. Before the exam, she reviewed her notes, got some sleep, and ate breakfast.

Items in a series should share the same grammatical form. This choice uses three past-tense verbs in a row: reviewed, got, ate.

Why the other choices are wrong:

• A. Before the exam, she reviewed her notes, got some sleep, and was eating breakfast. — Switches to was eating in the last item.
• B. Before the exam, she reviewed her notes, getting some sleep, and ate breakfast. — Switches to getting in the middle item.
• D. Before the exam, she was reviewing her notes, got some sleep, and to eat breakfast. — Mixes three different forms.

Source: Purdue OWL, Parallel Structure — Words and Phrases: matching grammatical forms.

Answer: D. adverb

Softly tells how the nurse spoke, so it describes a verb. Words that describe verbs are adverbs. By contrast, frightened is an adjective describing child.

Why the other choices are wrong:

• A. adjective — Adjectives describe nouns.
• B. noun — Softly doesn’t name a person, place, thing, or idea.
• C. verb — The verb in the sentence is spoke.

Source: Purdue OWL, Adjectives and Adverbs — The Basic Rules: Adverbs; adjectives versus adverbs.

Answer: D. Walking into the lab, I noticed the strong smell of disinfectant.

An opening phrase like “walking into the lab” describes the subject that comes right after it. Here, I is the one walking.

Why the other choices are wrong:

• A. Walking into the lab, the smell of disinfectant was strong. — The phrase seems to describe the smell, which can’t walk (a dangling modifier).
• B. The smell of disinfectant, walking into the lab, was strong. — The phrase still attaches to the smell.
• C. Walking into the lab, strong was the smell of disinfectant. — Same problem, with awkward word order.

Source: Purdue OWL, Dangling Modifiers — Main-clause subject must logically perform the introductory action.

Answer: B. Proximal

Proximal means closer to the trunk or to the point where a limb attaches. The elbow is closer to the shoulder than the wrist is, so the elbow is proximal to the wrist.

Why the other choices are wrong:

• A. Distal — Distal means farther from the trunk; the wrist is distal to the elbow.
• C. Lateral — Lateral means away from the body’s midline, not along a limb.
• D. Anterior — Anterior means toward the front of the body.

Source: NCI SEER Training, Anatomical Terminology — Directional Terms; Planes of the Body; Body Cavities.

Answer: C. medial

Medial means toward the body’s midline. The nose sits on the midline, while the ears are off to the sides, so the nose is medial to the ears.

Why the other choices are wrong:

• A. lateral — Lateral is the reverse relationship: the ears are lateral to the nose.
• B. distal — Distal describes position along a limb.
• D. posterior — Posterior means toward the back; the nose is on the front of the head.

Source: NCI SEER Training, Anatomical Terminology — Directional Terms; Planes of the Body; Body Cavities.

Answer: D. Sagittal plane

A sagittal plane runs vertically from front to back and divides the body into right and left sides. The median plane is the sagittal plane that runs exactly through the midline, creating equal halves.

Why the other choices are wrong:

• A. Coronal (frontal) plane — A coronal plane divides the body into front (anterior) and back (posterior) portions.
• B. Transverse (axial) plane — A transverse plane divides the body into upper and lower portions.
• C. Oblique plane — An oblique plane cuts at an angle rather than straight side to side or front to back.

Source: NCI SEER Training, Anatomical Terminology — Directional Terms; Planes of the Body; Body Cavities.

Answer: A. Diaphragm

The diaphragm, a dome-shaped breathing muscle, divides the ventral body cavity into the thoracic cavity above and the abdominopelvic cavity below.

Why the other choices are wrong:

• B. Sternum — The sternum (breastbone) is part of the front of the chest wall.
• C. Pericardium — The pericardium is the sac around the heart.
• D. Pleura — The pleura are membranes that line the chest cavity and cover the lungs.

Source: NCI SEER Training, Anatomical Terminology — Directional Terms; Planes of the Body; Body Cavities.

Answer: C. Esophagus

The thoracic (chest) cavity contains the heart, lungs, trachea, esophagus, large blood vessels, and nerves.

Why the other choices are wrong:

• A. Kidney — The kidneys are in the abdominal portion of the ventral cavity.
• B. Adrenal gland — The adrenal glands are also in the abdominal cavity.
• D. Urinary bladder — The bladder is in the pelvic cavity, with most of the urogenital system.

Source: NCI SEER Training, Anatomical Terminology — Directional Terms; Planes of the Body; Body Cavities.

Answer: B. Femur

The appendicular skeleton is made of the limb bones and the girdles that attach the limbs to the body. The femur (thigh bone) is a limb bone. SEER’s module counts 126 bones in the appendicular skeleton and 80 in the axial skeleton.

Why the other choices are wrong:

• A. Sternum — The sternum belongs to the axial skeleton, the body’s central axis.
• C. Vertebra — Vertebrae form the spine, part of the axial skeleton.
• D. Rib — Ribs are part of the axial skeleton.

Source: NCI SEER Training, Appendicular Skeleton — Shoulder girdle, upper and lower extremities; appendicular skeleton.

Answer: D. Pulmonary trunk

The pulmonary semilunar valve sits between the right ventricle and the pulmonary trunk. From the right ventricle, blood is pumped toward the lungs to pick up oxygen.

Why the other choices are wrong:

• A. Superior vena cava — The superior vena cava is one of the large systemic veins that returns blood to the right atrium.
• B. Aorta — The aorta receives blood from the left ventricle, through the aortic semilunar valve.
• C. Pulmonary veins — The pulmonary veins bring oxygenated blood from the lungs to the left atrium.

Source: NCI SEER Training, Structure of the Heart — Heart wall, chambers, valves and blood-flow pathway.

Answer: B. Bicuspid (mitral) valve

The left atrioventricular valve is the bicuspid valve, also called the mitral valve.

Why the other choices are wrong:

• A. Tricuspid valve — The tricuspid valve is the right atrioventricular valve.
• C. Pulmonary semilunar valve — The pulmonary semilunar valve is between the right ventricle and the pulmonary trunk.
• D. Aortic semilunar valve — The aortic semilunar valve is between the left ventricle and the aorta.

Source: NCI SEER Training, Structure of the Heart — Heart wall, chambers, valves and blood-flow pathway.

Answer: A. Left atrium

The atria are thin-walled receiving chambers. The left atrium receives oxygenated blood from the pulmonary veins; the right atrium receives deoxygenated blood from the systemic veins.

Why the other choices are wrong:

• B. Right atrium — The right atrium receives deoxygenated blood from the body.
• C. Right ventricle — The ventricles pump blood out of the heart rather than receive it from veins.
• D. Left ventricle — The left ventricle receives blood from the left atrium, not directly from the pulmonary veins.

Source: NCI SEER Training, Structure of the Heart — Heart wall, chambers, valves and blood-flow pathway.

Answer: C. Cardiac muscle

Cardiac muscle makes up the myocardium, the muscular layer of the heart wall. Like skeletal muscle it’s striated (striped), but like smooth muscle it works involuntarily.

Why the other choices are wrong:

• A. Skeletal muscle — Skeletal muscle moves the skeleton and is under voluntary control.
• B. Smooth muscle — Smooth muscle forms muscle layers in the walls of hollow organs such as the stomach and blood vessels, not the myocardium.
• D. Dense connective tissue — Dense connective tissue provides structural support; it is not the contracting muscle layer.

Source: NCI SEER Training, Muscle Types — Skeletal, Smooth and Cardiac Muscle paragraphs.

Related explanations: NCI SEER Training, Structure of the Heart — Heart wall, chambers, valves and blood-flow pathway.; NCI SEER Training, Connective Tissue — Structural support; dense connective tissue classification.

Answer: A. The brain and spinal cord

The central nervous system (CNS) is the brain and spinal cord. Peripheral ganglia and spinal nerves are part of the peripheral nervous system (PNS).

Why the other choices are wrong:

• B. Peripheral ganglia and spinal nerves — Peripheral ganglia and spinal nerves are part of the PNS.
• C. The brain and peripheral ganglia — The brain is part of the CNS, but peripheral ganglia are part of the PNS.
• D. The spinal cord and spinal nerves — Spinal nerves belong to the PNS.

Source: NCI SEER Training, Organization of the Nervous System — CNS and PNS paragraphs; voluntary/involuntary organization.

Answer: C. Alveoli

Alveoli are tiny air sacs with very thin walls surrounded by capillaries. Oxygen moves into the blood and carbon dioxide moves out across those walls.

Why the other choices are wrong:

• A. Bronchi — The bronchi are airways that carry air into the lungs.
• B. Trachea — The trachea (windpipe) carries air toward the bronchi.
• D. Larynx — The larynx (voice box) contains the vocal cords and conducts air; it is not a gas-exchange surface.

Source: NCI SEER Training, Bronchi, Bronchial Tree, & Lungs — Bronchial tree and alveolar gas-exchange paragraphs.

Related explanations: NCI SEER Training, Larynx & Trachea — Voice box, vocal cords and air conduction.

Answer: D. Liver

The liver makes bile, which helps break fats into smaller droplets so they can be digested. The gallbladder stores and concentrates bile but doesn’t make it.

Why the other choices are wrong:

• A. Gallbladder — The gallbladder stores bile made by the liver.
• B. Pancreas — The pancreas makes digestive enzymes and hormones, not bile.
• C. Stomach — The stomach secretes gastric juice.

Source: NCI SEER Training, Accessory Organs (digestive) — Liver; Gallbladder; Pancreas.

Answer: A. Nephron

Nephrons are the kidney’s functional units: they filter blood and form urine.

Why the other choices are wrong:

• B. Neuron — A neuron is a nerve cell.
• C. Alveolus — An alveolus is an air sac in the lung.
• D. Ureter — A ureter is a tube that carries urine from the kidney to the bladder.

Source: NIDDK, Your Kidneys & How They Work — How do my kidneys work? Nephrons, glomerulus and tubule; urine flow/ureter.

Related explanations: NCI SEER Training, Nerve Tissue — Opening definition of a neuron as a nerve cell.; NCI SEER Training, Bronchi, Bronchial Tree, & Lungs — Bronchial tree and alveolar gas-exchange paragraphs.

Answer: B. Insulin

Insulin, made by beta cells in the pancreatic islets, lowers blood glucose. Glucagon, from alpha cells, has the opposite effect and raises it.

Why the other choices are wrong:

• A. Glucagon — Glucagon raises blood glucose.
• C. Thyroxine — Thyroxine comes from the thyroid gland.
• D. Cortisol — Cortisol comes from the adrenal glands.

Source: OpenStax, Anatomy and Physiology 2e: The Endocrine Pancreas — 17.9: Cells and Secretions of Pancreatic Islets; Glucagon; Insulin; Table 17.7.

Related explanations: NCI SEER Training, Thyroid & Parathyroid Glands — Thyroid hormones thyroxine and triiodothyronine.; NCI SEER Training, Adrenal Gland — Cortical hormones, including cortisol.

Answer: B. are polar and attract charged ions

In a water molecule, oxygen pulls the shared electrons closer, giving the molecule a slightly negative end and slightly positive ends. Those charged ends surround the sodium and chloride ions and pull them apart.

Why the other choices are wrong:

• A. are nonpolar — Nonpolar is the opposite; oil is nonpolar, which is why it doesn’t mix with water.
• C. cannot form hydrogen bonds — Water molecules form hydrogen bonds with each other.
• D. have a very low specific heat — Water’s specific heat is high, and specific heat is about temperature change, not dissolving.

Source: OpenStax, Biology 2e: Water — Polarity, hydrogen bonding and high specific heat.

Answer: A. High specific heat

Water has a high specific heat: it takes a lot of energy to change its temperature, largely because hydrogen bonds between water molecules absorb energy first.

Why the other choices are wrong:

• B. Ice is less dense than liquid water — True, but it explains why ice floats on top of a lake, not slow temperature change.
• C. Water is a poor solvent — Water is an excellent solvent.
• D. Water molecules are nonpolar — Water molecules are polar.

Source: OpenStax, Biology 2e: Water — Polarity, hydrogen bonding and high specific heat.

Answer: D. Amino acids

Amino acids link together through peptide bonds to form proteins.

Why the other choices are wrong:

• A. Nucleotides — Nucleotides build nucleic acids such as DNA and RNA.
• B. Monosaccharides — Monosaccharides (simple sugars) build carbohydrates.
• C. Fatty acids and glycerol — Fatty acids and glycerol form many lipids.

Source: OpenStax, Biology 2e: Proteins — Amino Acids; peptide-bond formation.

Related explanations: OpenStax, Biology 2e: Carbohydrates — Monosaccharides and polysaccharides.; OpenStax, Biology 2e: Lipids — Fats and Oils: glycerol and fatty acids.; OpenStax, Biology 2e: Nucleic Acids — DNA/RNA nucleotide structure, standard bases, mRNA; DNA in organelles.

Answer: A. Uracil

RNA uses uracil where DNA uses thymine. Adenine, guanine, and cytosine appear in both.

Why the other choices are wrong:

• B. Thymine — Thymine is one of DNA’s four standard bases; RNA uses uracil in its corresponding standard set.
• C. Adenine — Adenine is found in both.
• D. Guanine — Guanine is found in both.

Source: OpenStax, Biology 2e: Nucleic Acids — DNA/RNA nucleotide structure, standard bases, mRNA; DNA in organelles.

Answer: B. By lowering the activation energy needed for the reaction to start

Enzymes are biological catalysts. They lower the activation energy, the energy barrier a reaction must get over, so the reaction goes faster.

Why the other choices are wrong:

• A. By supplying extra energy to the reactants — Enzymes don’t add energy to a reaction.
• C. By being used up as one of the reactants — Enzymes aren’t consumed; the same enzyme can be used again.
• D. By changing which products the reaction forms — An enzyme speeds up a reaction without changing its products.

Source: OpenStax, Biology 2e: Enzymes — Enzymes and activation energy.

Answer: C. A membrane-bound nucleus

A membrane-bound nucleus is characteristic of eukaryotic cells. Prokaryotic cells, such as bacteria, have DNA but no membrane-bound nucleus.

Why the other choices are wrong:

• A. A cell membrane — All cells have a cell (plasma) membrane.
• B. Ribosomes — Both cell types have ribosomes.
• D. DNA — Both cell types have DNA.

Source: OpenStax, Biology 2e: Eukaryotic Cells — Nucleus, nucleolus, ribosomes and mitochondria.

Related explanations: OpenStax, Biology 2e: Prokaryotic Cells — Components of Prokaryotic Cells; no membrane-bound nucleus.

Answer: B. Mitochondrion

Mitochondria carry out later stages of aerobic cellular respiration. Oxidative phosphorylation at the inner mitochondrial membrane produces most of the ATP in this setting.

Why the other choices are wrong:

• A. Golgi apparatus — The Golgi apparatus modifies, sorts, and packages proteins.
• C. Lysosome — Lysosomes break down worn-out materials.
• D. Nucleolus — The nucleolus makes ribosome components.

Source: OpenStax, Biology 2e: Oxidative Phosphorylation — Electron transport, inner mitochondrial membrane and ATP synthesis.

Related explanations: OpenStax, Biology 2e: Eukaryotic Cells — Nucleus, nucleolus, ribosomes and mitochondria.; OpenStax, Biology 2e: The Endomembrane System and Proteins — Golgi Apparatus; Lysosomes.

Answer: A. Messenger RNA (mRNA)

DNA in the nucleus directs protein synthesis, and messenger RNA carries that genetic information out to the cytoplasm, where proteins are made.

Why the other choices are wrong:

• B. ATP — ATP carries energy, not genetic information.
• C. Glucose — Glucose is a sugar used for energy.
• D. Phospholipids — Phospholipids build membranes.

Source: OpenStax, Biology 2e: Nucleic Acids — DNA/RNA nucleotide structure, standard bases, mRNA; DNA in organelles.

Related explanations: OpenStax, Biology 2e: Eukaryotic Cells — Nucleus, nucleolus, ribosomes and mitochondria.

Answer: D. simple diffusion

Simple diffusion moves particles down their concentration gradient, from higher concentration to lower concentration.

Why the other choices are wrong:

• A. active transport — Active transport moves substances the other way, from lower to higher concentration, using energy.
• B. exocytosis — Exocytosis releases material when a vesicle fuses with the cell membrane.
• C. filtration — Filtration uses pressure to push substances through a membrane.

Source: OpenStax, Biology 2e: Passive Transport — Diffusion versus facilitated transport; Osmosis; Tonicity and hypertonic cells.

Related explanations: OpenStax, Biology 2e: Bulk Transport — Exocytosis: vesicle fusion and release.; NCI SEER Training, Cell Function — Membrane transport; Cell Division; DNA and protein synthesis.

Answer: D. Water

Osmosis is the diffusion of water (or another solvent) through a selectively permeable membrane.

Why the other choices are wrong:

• A. Glucose — Glucose movement isn’t osmosis.
• B. Proteins — Protein movement isn’t osmosis.
• C. Oxygen only — Oxygen moves by simple diffusion.

Source: OpenStax, Biology 2e: Passive Transport — Diffusion versus facilitated transport; Osmosis; Tonicity and hypertonic cells.

Answer: C. It moves substances from lower to higher concentration and requires energy and a carrier molecule.

Active transport moves substances against their concentration gradient, from lower to higher concentration. That requires a carrier molecule and energy.

Why the other choices are wrong:

• A. It moves only water across a membrane. — That describes osmosis.
• B. It moves substances from higher to lower concentration without using energy. — That describes diffusion.
• D. It uses pressure to push fluid through a membrane. — That describes filtration.

Source: OpenStax, Biology 2e: Active Transport — Energy and transport against an electrochemical gradient.

Answer: B. Water moves out of the cell, and the cell shrinks.

Water moves by osmosis toward the side with more nonpenetrating solute particles. The outside solution is hypertonic to the cell, so water leaves the cell and the cell shrinks.

Why the other choices are wrong:

• A. Water moves into the cell, and the cell swells. — That happens in a hypotonic solution, which has a lower concentration of nonpenetrating solute particles than the cell.
• C. Nothing changes, because the membrane blocks water. — Cell membranes let water pass; that’s what makes osmosis possible.
• D. Solutes are pumped out, and the cell swells. — This mixes up solute pumping with osmosis and gets the direction wrong.

Source: OpenStax, Biology 2e: Passive Transport — Diffusion versus facilitated transport; Osmosis; Tonicity and hypertonic cells.

Answer: C. 23

Meiosis halves the chromosome number. Typical nucleated body (somatic) cells have 46 chromosomes, so eggs and sperm have 23. At fertilization, 23 + 23 restores 46.

Why the other choices are wrong:

• A. 46 — 46 is the usual chromosome number in a nucleated human body cell.
• B. 92 — 92 would double the normal number.
• D. 12 — 12 isn’t the human gamete number.

Source: NCI SEER Training, Cell Function — Membrane transport; Cell Division; DNA and protein synthesis.

Answer: A. Cytoplasm

Glycolysis, the first stage of cellular respiration, splits glucose into two pyruvate molecules in the cytoplasm. In eukaryotic cells, later stages of aerobic respiration take place in the mitochondria.

Why the other choices are wrong:

• B. Nucleus — The nucleus holds DNA; glycolysis doesn’t happen there.
• C. Mitochondrial matrix — The citric acid cycle happens in the mitochondrial matrix, after glycolysis.
• D. Chloroplast — Chloroplasts carry out photosynthesis.

Source: OpenStax, Biology 2e: Glycolysis — Opening location in cytoplasm; glucose-to-pyruvate stages.

Related explanations: OpenStax, Biology 2e: Oxidation of Pyruvate and the Citric Acid Cycle — Mitochondrial matrix; oxygen dependence without direct consumption by the cycle.; OpenStax, Biology 2e: Overview of Photosynthesis — Basic process, carbon dioxide, water, carbohydrate and oxygen.

Answer: D. Glucose (a sugar) and oxygen

Photosynthesis uses light energy to turn carbon dioxide and water into sugar (glucose), releasing oxygen.

Why the other choices are wrong:

• A. Carbon dioxide and water — Carbon dioxide and water are the reactants, not the products.
• B. Oxygen and carbon dioxide — Carbon dioxide is taken in, not produced.
• C. Glucose and nitrogen — Nitrogen isn’t a product of photosynthesis.

Source: OpenStax, Biology 2e: Overview of Photosynthesis — Basic process, carbon dioxide, water, carbohydrate and oxygen.

Answer: C. An iron nail rusting

In a chemical change, substances become new substances with different compositions. Rust (an iron oxide) forms when iron reacts with oxygen in the presence of water.

Why the other choices are wrong:

• A. Ice melting into water — Melting changes water’s state, not its identity.
• B. Sugar dissolving in tea — Dissolving mixes sugar into the tea; the sugar molecules are unchanged.
• D. A glass breaking — Breaking changes the glass’s shape, not what it’s made of.

Source: OpenStax, Chemistry 2e: Physical and Chemical Properties — Physical changes versus chemical reaction; rusting.

Answer: A. 17

The atomic number is the number of protons, so it’s 17 (chlorine). The mass number would be protons + neutrons = 35.

Why the other choices are wrong:

• B. 18 — 18 is the number of neutrons.
• C. 34 — Adds protons and electrons, which doesn’t give any standard quantity.
• D. 35 — 35 is the mass number.

Source: OpenStax, Chemistry 2e: Atomic Structure and Symbolism — Atomic number, mass number, protons/neutrons/electrons, ions and isotopes.

Answer: B. 8

The 14 in carbon-14 is the mass number: protons + neutrons. Neutrons = 14 − 6 = 8.

Why the other choices are wrong:

• A. 6 — 6 is the number of protons.
• C. 14 — 14 is the mass number.
• D. 20 — Adds the mass number and the protons.

Source: OpenStax, Chemistry 2e: Atomic Structure and Symbolism — Atomic number, mass number, protons/neutrons/electrons, ions and isotopes.

Answer: C. neutrons

Isotopes have the same number of protons, which is what makes them the same element, but different numbers of neutrons, so their mass numbers differ.

Why the other choices are wrong:

• A. protons — A different number of protons would make a different element.
• B. electrons, when the atoms are neutral — Neutral atoms of the same element have the same number of electrons as protons.
• D. chemical symbols — Isotopes of an element share its chemical symbol.

Source: OpenStax, Chemistry 2e: Atomic Structure and Symbolism — Atomic number, mass number, protons/neutrons/electrons, ions and isotopes.

Answer: D. +1

Charge = protons − electrons = 11 − 10 = +1. With one more proton than electrons, it’s a positive ion (a cation). This is a sodium ion, Na⁺.

Why the other choices are wrong:

• A. −1 — Reverses the subtraction.
• B. 0 — A neutral atom would need equal numbers of protons and electrons.
• C. +10 — Counts the electrons as the charge.

Source: OpenStax, Chemistry 2e: Atomic Structure and Symbolism — Atomic number, mass number, protons/neutrons/electrons, ions and isotopes.

Answer: B. valence electrons

These main-group elements in a group have the same number of valence (outermost) electrons, which largely determines how they react.

Why the other choices are wrong:

• A. neutrons — Neutron numbers vary among the elements in a group.
• C. protons — Each element has its own number of protons.
• D. occupied energy levels — Elements in the same period (row), not group, share the same number of occupied energy levels.

Source: OpenStax, Chemistry 2e: Electronic Structure of Atoms — Valence Electrons and the Periodic Table; main-group scope and helium configuration.

Related explanations: OpenStax, Chemistry 2e: The Periodic Table — Groups/periods, element listing and group 18 noble gases.

Answer: D. Neon

Neon is in Group 18, the noble gases, which are very unreactive.

Why the other choices are wrong:

• A. Sodium — Sodium is an alkali metal (Group 1).
• B. Chlorine — Chlorine is a halogen (Group 17).
• C. Oxygen — Oxygen is in Group 16.

Source: OpenStax, Chemistry 2e: The Periodic Table — Groups/periods, element listing and group 18 noble gases.

Answer: A. Ionic bond

Sodium transfers an electron to chlorine, forming Na⁺ and Cl⁻ ions. The attraction between these oppositely charged ions is an ionic bond.

Why the other choices are wrong:

• B. Nonpolar covalent bond — Covalent bonds share electrons rather than transfer them.
• C. Hydrogen bond — Hydrogen bonds can form between water molecules; they do not hold the Na⁺ and Cl⁻ lattice together.
• D. Metallic bond — Metallic bonding holds pure metals together.

Source: OpenStax, Chemistry 2e: Ionic Bonding — NaCl and electrostatic attraction between opposite ions.

Related explanations: OpenStax, Biology 2e: Water — Polarity, hydrogen bonding and high specific heat.; OpenStax, Chemistry 2e: The Solid State of Matter — Metallic Solids.

Answer: C. Polar covalent bond

The atoms share electrons, which makes the bond covalent, and oxygen pulls the shared electrons closer, which makes it polar.

Why the other choices are wrong:

• A. Ionic bond — No electrons are fully transferred.
• B. Nonpolar covalent bond — Nonpolar bonds share electrons equally.
• D. Hydrogen bond — A hydrogen bond is an attraction between separate molecules, not the bond inside one water molecule.

Source: OpenStax, Chemistry 2e: Covalent Bonding — Polar Covalent Bonds; unequal sharing and partial charges.

Related explanations: OpenStax, Biology 2e: Water — Polarity, hydrogen bonding and high specific heat.

Answer: D. 2, 1, 2

With 2H₂ + O₂ → 2H₂O, each side has 4 hydrogen atoms and 2 oxygen atoms.

Why the other choices are wrong:

• A. 1, 1, 1 — Leaves 2 oxygen atoms on the left and only 1 on the right.
• B. 1, 2, 2 — Leaves 2 hydrogen atoms on the left and 4 on the right, and 4 oxygen atoms on the left but 2 on the right.
• C. 2, 2, 2 — Balances hydrogen but leaves 4 oxygen atoms on the left and 2 on the right.

Source: OpenStax, Chemistry 2e: Writing and Balancing Chemical Equations — Methane combustion, coefficients, equal atom counts and conservation.

Answer: A. Atoms are not created or destroyed in a chemical reaction.

Mass is conserved: a reaction rearranges atoms, so each element needs the same number of atoms on both sides.

Why the other choices are wrong:

• B. The molecules on each side must be identical. — Reactions turn reactants into different products.
• C. Every reaction must produce water. — Many reactions produce no water.
• D. The coefficients must add up to the same total on both sides. — The number of molecules can change: 2H₂ + O₂ → 2H₂O has 3 molecules on the left and 2 on the right.

Source: OpenStax, Chemistry 2e: Writing and Balancing Chemical Equations — Methane combustion, coefficients, equal atom counts and conservation.

Answer: B. CO₂ and H₂O

Products are written to the right of the arrow; reactants are on the left. Here methane reacts with oxygen to produce carbon dioxide and water.

Why the other choices are wrong:

• A. CH₄ and O₂ — These are the reactants.
• C. CH₄ and CO₂ — Mixes a reactant with a product.
• D. O₂ and H₂O — Mixes a reactant with a product.

Source: OpenStax, Chemistry 2e: Writing and Balancing Chemical Equations — Methane combustion, coefficients, equal atom counts and conservation.

Answer: B. pH 2

A lower pH means a higher concentration of hydrogen (hydronium) ions, so pH 2 is the most acidic choice. Each whole pH unit is a tenfold change, so pH 2 corresponds to 1,000 times the hydronium-ion concentration at pH 5.

Why the other choices are wrong:

• A. pH 5 — pH 5 is acidic, but less acidic than pH 2.
• C. pH 7 — At 25 °C, pH 7 is neutral.
• D. pH 11 — pH 11 is basic.

Source: OpenStax, Chemistry 2e: pH and pOH — pH logarithm and dilute hydronium concentration; neutrality at 25 °C.

Answer: A. Sodium chloride and water

This is a neutralization reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The acid’s H⁺ and the base’s OH⁻ form water; sodium and chloride remain dissolved as spectator ions.

Why the other choices are wrong:

• B. Hydrogen gas and oxygen gas — Neither gas is produced in this reaction.
• C. Chlorine gas and sodium metal — Neither element is produced in this reaction.
• D. Sodium hydroxide and water — Sodium hydroxide is a reactant consumed in the reaction, not a product.

Source: OpenStax, Chemistry 2e: Classifying Chemical Reactions — Acid-Base Reactions; HCl/NaOH and net ionic neutralization.

Answer: D. 10 g

18 hours is three half-lives (18 ÷ 6 = 3). Halve the amount of the original isotope three times: 80 → 40 → 20 → 10 g.

Why the other choices are wrong:

• A. 40 g — Stops after one half-life.
• B. 26.7 g — Divides 80 by 3 instead of halving three times.
• C. 20 g — Stops after two half-lives.

Source: OpenStax, Chemistry 2e: Radioactive Decay — Half-life and number of undecayed nuclei remaining.