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.