请教OG11-43-66？

minicai

the content of passge:

There were some apparent exceptions to the general

correlation observed between adenosine-receptor binding

and stimulation. One of these was a compound called

3-isobuty1-1-methylxanthine(IBMX), which bound very

(50) well but actually depressed mouse locomotion. Snyder

et al suggest that this is not a major stumbling block to

their hypothesis. The problem is that the compound has

mixed effects in the brain, a not unusual occurrence with

psychoactive drugs. Even caffeine, which is generally

(55) known only for its stimulatory effects, displays this

property, depressing mouse locomotion at very low

concentrations and stimulating it at higher ones.

请问下题为什么选A，能帮我翻译一下上段内容

In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon for psychoactive drugs to have

(A)  mixed effects in the brain

(B)  inhibitory effects on enzymes in the brain

(C)  close structural relationships with caffeine

(D)  depressive effects on mouse locomotion

(E)   the ability to dislodge caffeine from receptors in the brain