What Keeps a Nerve Action Potential from Flowing "Backward" in a Neuron

Multiple Choice

What keeps a nerve action potential from flowing "backward" in a neuron?

A) Schwann cells keep action potentials flowing one direction
B) Sodium ions can only move toward a synapse
C) Chemical synapse receptors form a one-way gate and there is a refractory period after sodium ions are pumped back out
D) The axon and dendrite have completely different membrane structures
E) All of the choices are correct