This is a Paradox question, in which you are asked to explain the facts presented in the stimulus. This stimulus consists of a set of facts (no conclusion, so not an argument). You are given certain information about plaque buildup on teeth, and then asked to select the answer choice that "most helps to explain the relative amounts of plaque buildup found in the three groups."
Answer choice (B) says "the stiffness of the bristles on new toothbrushes, which the mechanical action of brushing destroys after several months, inhibits the buildup of plaque." This explains why Group 3 (the group that got a brand-new toothbrush every month) had less plaque buildup than Group 1 (same toothbrush all year = less stiff bristles over time) or Group 2 (same toothbrush all year; even though sterilized each month, bristles less stiff over time). This is the correct answer.
Answer choice (A) is incorrect because it doesn't explain why the outcome is the same for Groups 1 and 2. Remember: Group 2 participants *did* sterilize their toothbrushes every month. So why do Group 1 and Group 2 participants have the same amount of plaque buildup, and Group 3 participants have less plaque? This answer does not answer these questions, so it is not correct.
Hope this helps! Please let us know if you have any additional questions.
