Answer C - In this argument, the conclusion is that people should change their motor oil every 3,000 miles. The conclusion is based on the fact that motor oil loses half of its effectiveness after being driven that distance. The central assumption here is that the loss of effectiveness is indeed a valid reason for changing the motor oil. Choice (C) challenges this assumption by saying that cars with motor oil at fifty percent effectiveness do not have any ill effects. Thus, choice (C) is the correct answer.

Choices (A), (B), and (E) are all out of scope. The fact that oil loses effectiveness because it becomes dirty does not weaken the conclusion. It also does not matter that cars that reach 3,000 miles at different rates experience similar declines in motor oil effectiveness; it also does not matter that the same is true of cars with different types of motor oil.

Choice (D) may be tempting, but is ultimately irrelevant. We may want to gravitate to a choice like (D) that questions the motives for the research, however we never want to challenge the evidence itself in a weaken question – only the central assumption. Even though the research was conducted under the supervision of motor oil companies, this does not alone weaken the conclusion that motor oil should be changed every 3,000 miles.