Why is answer D not also correct?

I didn't choose answer choice A because besides the rat analogy, the passage didn't discuss anything about protein-rich snacks. Answer choice D, however, is directly supported by the passage. In lines 22-25 it states "Blood tryptophan levels, however, are unaffected by insulin, so the proportion of tryptophan in the blood relative to the other amino acids increases when carbs are consumed." Well, in the question stems, if a drug is given that inhibits seratonin, it would cause the subject to crave more carbs. In doing so, more carbs would be consumed and therefore the blood tryptophan levels would also increase.