Derek Has Been Asked to Hand-Deliver a Message to Adeline

Derek has been asked to hand-deliver a message to Adeline. Adeline lives across the river from Derek, and there is no bridge. Derek's friends Hakeem, Geoff, and Rion have also been asked to carry messages across the river. The four friends agree that they will share a boat across the channel so they can deliver their messages. If we compare this scenario to neuron communication, we can say that the boat, which carries the messengers, is like a

A) synaptic vesicle.
B) neurotransmitter.
C) synapse.
D) receptor site.