Section IV 22

22.   In humans, ingested protein is broken down into amino acids, all of which must compete to enter the brain. Subsequent ingestion of sugars leads to the production of insulin, a hormone that breaks down the sugars and also rids the bloodstream of residual amino acids, except for tryptophan, Tryptophan then slips into the brain uncontested and is transformed into the chemical serotonin, increasing ht brain’s serotonin level. Thus sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free.

Which one of the following is an assumption on which the argument depends?

(A) Elevation of mood and freedom from anxiety require increasing the level of serotonin the brain.

(B) Failure to consume foods rich in sugars results in anxiety and a lowering of mood.

(C) Serotonin can be produced naturally only if tryptophan is presented in the bloodstream.

(D) Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety.

(E) The consumption of protein-rich foods results in anxiety and a lowering of mood.

答案 D

问题A选项如何排除，求教