I don't understand why C is the answer

Thank you for your question. The question stem asks us to strengthen the argument in the stimulus, so let's first be sure that we have a good grasp on what that argument is. The conclusion in the stimulus is that melatonin is not necessarily helpful in treating insomnia. The premises given in support of that conclusion are: Premise: many scientific studies have suggested that taking melatonin tablets can induce sleep Premise: most of the studies examined only people without insomnia Premise: in many of the studies, only a few of the subjects given melatonin appeared to be significantly affected by it. All right. Now let's look at the correct answer choice. Answer choice (C) says that "in the studies that included subjects with insomnia, only subjects without insomnia were significantly affected by doses of melatonin." This would actually help strengthen the conclusion in the stimulus, that is, that melatonin is not necessarily helpful in treating insomnia. You question whether answer choice (C) does not weaken the argument, which makes me think that you actually misread the stimulus or lost track of the conclusion you were actually trying to strengthen. Anyway, hope this helps! Please let us know if you have any other questions.

Why is B incorrect? Isn't this attacking the representativeness of the sample?