Answer choice A does not go far enough in strengthening the idea that melatonin isn't helpful for treating insomnia. First of all, a weaker correlation does not necessarily mean a weak correlation; we don't know what the original correlation even is. Secondly, there's not any information that says the weaker correlation was due to the fact that subjects with insomnia were included in the study. A number of factors could have accounted for any change that was seen and assuming that the weaker correlation was due to the presence of people with insomnia in the study is an unwarranted assumption.
C is correct on the other hand because it directly tells us that people with insomnia were not affected by the melatonin in studies in which they were included. This eliminates the possibility that the subjects talked about in the stimulus that were significantly affected were subjects with insomnia.
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C is correct on the other hand because it directly tells us that people with insomnia were not affected by the melatonin in studies in which they were included. This eliminates the possibility that the subjects talked about in the stimulus that were significantly affected were subjects with insomnia.
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