When Neurotransmitters Are Being Released by a Presynaptic Neuron into the Synapse,it

Multiple Choice

 When neurotransmitters are being released by a presynaptic neuron into the synapse,it is because ____. 

A) the neuron is in a refractory period
B) there is a large concentration of Na+ ions outside of the presynaptic axon
C) action potentials have occurred in the postsynaptic neuron
D) action potentials have occurred in the presynaptic neuron