The primary purpose of the second paragraph in the passage is to A. provide an overview of current research concerning the effect of serotonin on carbohydrate consumption B. contrast the role of tryptophan in the body with that of serotonin C. discuss the role of serotonin in the transmission of neural impulses D. explain how the brain knows that carbohydrates should be consumed E. establish a connection between carbohydrate intake and the production of serotonin