When voltage-gated calcium ion channels open at the synaptic terminal:
a. calcium ions exit the cell, which repolarizes the membrane and resets the resting membrane potential.
b. calcium ions rush into the neuron, depolarizing the membrane and triggering an action potential.
c. calcium ions rush into the presynaptic neuron, which causes synaptic vesicles to release neurotransmitter into the synaptic cleft.
d. voltage-gated sodium channels on the postsynaptic neuron open, which triggers an action potential if threshold is reached