Harry Is Going to Mei's House to Feed Mei's Cat

Harry is going to Mei's house to feed Mei's cat while she is gone. Mei left Harry her keyring, but when Harry reaches the front door, he realizes he doesn't know which key fits the lock. He tries several before finding the right one. If we compare the key and the lock to the behavior of communicating neurons, we can say that the key is like a __________ and the lock is like a

A) vesicle; synaptic space.
B) neurotransmitter; receptor site.
C) presynaptic neuron; vesicle.
D) neurotransmitter; Node of Ranvier.