This question is great because it points you to the part of the question at issue (lines 15-31). However, this is also what makes it tricky as to get the right answer you have to read that paragraph in relation to the rest of the passage.
(A) is incorrect. The paragraph does not mention current research.
(B) is incorrect. While this paragraph discusses the relationship between tryptophan and serotonin, it does not "contrast them." Rather, the explains the process whereby tryptophan is converted into serotonin. This is tricky, because this final "conversion" does not occur until the last sentence of the paragraph (see line 31).
(C) is incorrect. There is no discussion of neural impulses in this paragraph.
(D) is incorrect. This answer choice refers to paragraph 3 not paragraph 2.
(E) is correct. Lines 17-19 state "the rate of conversion is affected by the production of carbohydrates in an individuals diet." Lines 29-31 state "speed tryptophan's entry into the central nervous system where.... it is converted into serotonin." This paragraph explains all the steps than occur between an individual consuming carbohydrates and the production of serotonin. Thus, it establishes a connection between the two actions.
