Question

a population of bacteria may evolve resistance to a drug over time. this graph shows the growth of a bacterial population before and after the introduction of an antibiotic, with some bacteria becoming resistant to the antibiotic. graph with a, b, c, d (time points) and population size (y - axis)
part a - interpreting the graph
which time point on the graph shows when the antibiotic was first added?
options: a, b, c, d
correct explanation: the addition of an antibiotic would kill most of the bacteria in a population, causing the population size to decline rapidly. this is the case at time point d.
part b - thinking critically
which three statements may correctly explain why the population size increases after time point c?
options:

• between time points c and d, drug - resistant bacteria were reproducing faster than non - drug - resistant bacteria were dying.
• the population increase just after time point c indicates that antibiotic use was discontinued.
• the few drug - resistant bacteria in the population reproduced, quickly leading to a large drug - resistant population.
• because the population grew more rapidly after time point c, the bacteria must have acquired a second drug - resistance mutation.
• bacteria that acquired a mutation that conferred drug resistance had a growth advantage over non - resistant bacteria.

Explanation:

Part A

Step1: Analyze antibiotic effect

Antibiotic kills bacteria, so population should drop rapidly when added.

Step2: Match with graph points

Time point B: Population starts to decline rapidly, indicating antibiotic addition. Wait, no—wait, the correct explanation says D? Wait, no, recheck. Wait, the graph: before antibiotic, population grows, then antibiotic added, population drops. Wait, the initial graph: A is early, B maybe a peak, then C is the low point? Wait, no, the correct answer explanation says "the addition of an antibiotic would kill most of the bacteria... causing population to decline rapidly. This is the case at time point B"? Wait, no, the user's correct answer shows "Correct: The addition of an antibiotic... at time point B"? Wait, maybe I misread. Wait, the graph: x-axis time, y-axis population. When antibiotic is added, population should decrease. So the time point where population starts to drop rapidly is when antibiotic is added. So if the graph has a peak at B, then drops? Wait, the user's correct answer says time point B? Wait, no, the user's "Problem Answer" section has a checkmark on B? Wait, the correct explanation says "the addition of an antibiotic would kill most of the bacteria in a population, causing the population size to decline rapidly. This is the case at time point B". So the answer is B.

1. Statement 1: Between C and D, drug - resistant bacteria reproduce faster than non - drug - resistant die. Drug - resistant bacteria survive antibiotic, so if they reproduce faster than non - resistant die, population can increase. Correct.
2. Statement 2: Population increase after C doesn't mean antibiotic was discontinued. Antibiotic may still be present, but resistant bacteria are growing. Incorrect.
3. Statement 3: Few drug - resistant bacteria can reproduce quickly (exponential growth) to form a large population. Correct, as resistant bacteria have a survival advantage and can multiply.
4. Statement 4: Faster growth after C doesn't mean a second mutation. Resistant bacteria from first mutation can grow. Incorrect.
5. Statement 5: Drug - resistant bacteria (with mutation) have a growth advantage over non - resistant (as non - resistant are killed by antibiotic), so they can grow better. Correct.

So the three correct statements are:

• Between time points C and D, drug - resistant bacteria were reproducing faster than non - drug - resistant bacteria were dying.
• The few drug - resistant bacteria in the population reproduced, quickly leading to a large drug - resistant population.
• Bacteria that acquired a mutation that conferred drug - resistance had a growth advantage over non - resistant bacteria.

Answer:

B

Part B