![](https://cos-cdn.shuashuati.com/pipixue-wap/2020-1230-1107-56/ti_inject-812ce.png)
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.