Why Is an Action Potential Propagated Down an Unmyelinated Neuron

Why is an action potential propagated down an unmyelinated neuron plasma membrane?

A) because the action potential stimulates voltage-gated Cl^- channels adjacent to it
B) because the action potential stimulates voltage-gated Ca²⁺ channels adjacent to it
C) because the action potential stimulates voltage-gated Mg⁺ channels adjacent to it
D) because the action potential stimulates voltage-gated Na⁺ channels adjacent to it