Caffeine is a ___ that affects the brain stem by ___.
A. stimulant; blocking the reuptake of the neurotransmitter adenosine. B. depressant; blocking the reuptake of the neurotransmitter adenosine. C. stimulant; aiding in the reuptake of the neurotransmitter adenosine. D. depressant; aiding in the reuptake of the neurotransmitter adenosine.