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.
