GMAT阅读理解样题
Answer with explanation:
　　D is the best answer. This question requires you to identify the primary concern of the passageas a whole. The first paragraph presents a recent hypothesis about how caffeine affectsbehavior. The second paragraph describes an earlier and widely accepted hypothesis abouthow caffeine affects behavior, and then presents evidence that is not consistent with thathypothesis. The third and fourth paragraphs return to the newer hypothesis introduced in thefirst paragraph and provide "evidence and arguments" that support this alternative hypothesis.