Each of the Following Contributes to the Unidirectional Propagation of Nerve

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.