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
　  (C) ratio of the rat's blood-tyrosine concentration to its blood-leucine concentration

　　(D) number of neurotransmitters of any kind that the rat will produce and release

　　(E) number of amino acids the rat's blood will contain

　　6. The authors' discussion of the "mechanism that provides blood tryptophan to the brain cells" (lines 31-32) is meant to

　　(A) stimulate further research studies

　　(B) summarize an area of scientific investigation

　　(C) help explain why a particular research finding was obtained

　　(D) provide supporting evidence for a controversial scientific theory

　　(E) refute the conclusions of a previously mentioned research study

　　7. According to the passage, an injection of insulin was most similar in its effect on rats to an injection of

　　(A) tyrosine

　　(B) leucine

　　(C) blood

　　(D) tryptophan

　　(E) protein

　　8. It can be inferred from the passage that which of the following would be LEAST likely to be a potential source of aid to a patient who was not adequately producing and releasing serotonin?

　　(A) Meals consisting almost exclusively of protein

　　(B) Meals consisting almost exclusively of carbohydrates

　　(C) Meals that would elicit insulin secretion

　　(D) Meals that had very low concentrations of tyrosine

　　(E) Meals that had very low concentrations of leucine

　　9. It can be inferred from the passage that the authors initially held which of the following hypotheses about what would happen when they fed large amounts of protein to rats?

　　(A) The rats' brain serotonin levels would not decrease.

　　(B) The rats' brain tryptophan levels would decrease　

      (C) The rats' tyrosine levels would increase less quickly than would their leucine levels

　　(D) The rats would produce more insulin.

　　(E) The rats would produce neurotransmitters other than serotonin.