Why (D) is right: ruling out an alternative explanation
A Strengthen explanation from our free library. The question below is original, written for this page. Then the full breakdown: why the right answer is right, and why each wrong answer is wrong.
The question
Office workers who keep plants on their desks report higher job satisfaction than workers without plants. A researcher concludes that having a plant improves morale. A critic replies that people who bother to keep plants may simply be friendlier people to begin with.
Which one of the following, if true, most strengthens the researcher's conclusion?
- Workers with plants also report taking fewer sick days.
- Most workers who keep plants also keep family photos on their desks.
- The study surveyed more than ten thousand workers.
- The satisfaction gap remained even after accounting for workers' friendliness scores measured before anyone got a plant. Correct
- Job satisfaction surveys are a reliable way to measure morale.
Why (D) is correct
The critic offered a rival story: friendlier people keep plants and also report more satisfaction, so the plants do nothing. Choice (D) tests that story directly. It looks at workers who were equally friendly before plants entered the picture and finds the gap still there.
If friendliness explained the result, the gap should have vanished once friendliness was held constant. It did not. That leaves the plant explanation standing stronger. This is the classic strengthen move: kill the best alternative explanation.
Why the wrong answers are wrong
(A) Out of scope
Sick days are about health, not morale. The conclusion is about job satisfaction, so health data does not support it.
(B) True but irrelevant
The photos might matter, but this gives no reason to think the plants do. It adds a fact without connecting it to the conclusion.
(C) Half-right
A bigger sample makes the correlation more solid, but the critic never attacked the sample size. The objection was about who chooses to keep plants. A large biased sample is still biased.
(E) True but irrelevant
This defends the measurement, but nobody attacked the measurement. The fight is about what explains the gap, not how morale was measured.
Last updated October 12, 2026