How Can Synaptic Vesicles Continue to Pour Out Their Neurotransmitters

How can synaptic vesicles continue to pour out their neurotransmitters, and yet still have a ready supply of neurotransmitters to respond to continuing stimulation?

A) Through the process of reuptake, they absorb some of the neurotransmitter left the gap between the axon and the dendrite
B) Other nearby cells donate a portion of neurotransmitter to the vesicle need, through a process called, "synaptic transfer."
C) The vesicles never fire completely, always holding a little amount of neurotransmitter back.
D) The cells re-energize with every firing.