Question ID:PT38 S4 Q22 Passage: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 the brain's serotonin level. Thus, sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free. Stem:Which one of the following is an assumption on which the argument depends? Correct Answer Choice:DChoice A:Elevation of mood and freedom from anxiety require increasing the level of serotonin in the brain. Choice B:Failure to consume foods rich in sugars results in anxiety and a lowering of mood.  Choice C:Serotonin can be produced naturally only if tryptophan is present in the bloodstream. Choice D:Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety. Choice E:The consumption of protein-rich foods results in anxiety and a lowering of mood.