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发布时间：2006-06-23 17:04     点击：

　　2. According to the passage, the speed with which tryptophan is provided to the brain cells of a rat varies with the

　　(A) amount of protein present in a meal

　　(B) concentration of serotonin in the brain before a meal

　　(C) concentration of leucine in the blood rather than on the concentration of tyrosine in the blood after a meal

　　(D) concentration of tryptophan in the brain before a meal

　　(E) number of serotonin-containing neurons present in the brain before a meal

　　3. According to the passage, when the authors began their first studies, they were aware that

　　(A) they would eventually need to design experiments that involved feeding rats high concentrations of protein

　　(B) tryptophan levels in the blood were difficult to monitor with accuracy

　　(C) serotonin levels increased after rats were fed meals rich in tryptophan

　　(D) there were many neurotransmitters whose production was dependent on metabolic processes elsewhere in the body.

　　(E) serotonin levels increased after rats were injected with a large amount of tryptophan

　　4. According to the passage, one reason that the authors gave rats carbohydrates was to

　　(A) depress the rats' tryptophan levels

　　(B) prevent the rats from contracting diseases

　　(C) cause the rats to produce insulin

　　(D) demonstrate that insulin is the most important substance secreted by the body

　　(E) compare the effect of carbohydrates with the effect of proteins

　　5. According to the passage, the more protein a rat consumes, the lower will be the

　　(A) ratio of the rat's blood-tryptophan concentration to the amount of serotonin produced and released in the rat's brain

　　(B) ratio of the rat's blood-tryptophan concentration to the concentration in its blood of the other amino acids contained in the protein

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