Question ID:PT93 S3 Q25 Passage:To test the claim that vitamin C is effective in treating acne, scientists administered it to one group of subjects and a placebo to a control group. The group receiving vitamin C had less severe acne during the study than did the control group. It was subsequently discovered, however, that half of the subjects in each group knew which kind of pill they were given. Among those who could not tell, no difference in the severity of acne was found between the two groups. Therefore, we can tentatively conclude that vitamin C has no real benefit in reducing the severity of acne. Stem:Which one of the following, if true, most seriously weakens the argument? Correct Answer Choice:AChoice A:The subjects who were given vitamin C had a history of suffering from more severe acne than did the subjects receiving a placebo. Choice B:None of the subjects who were given vitamin C took additional doses of vitamin C on their own. Choice C:During the study, the severity of the subjects' acne was lower than the national average. Choice D:Some of the subjects who were given placebos consumed foods during the study that are naturally rich in vitamin C. Choice E:Some of the subjects who knew their pills were placebos did not actually take the pills they were given.