Each of the following contributes to the unidirectional propagation of nerve signals except

A)dendrites only receive signals.
B)presynaptic neurotransmitter secretion is separated from postsynaptic receptors.
C)neuron membranes have a refractory period following generation of an action potential.
D)ion channels are oriented to transport ions only into the neuron.
E)action potentials initiate the hillock and propagate only down the axon.