70. Action potentials are usually propagated in only one direction along an axon because(A)ions can flow along the axon only in one direction.(B) the brief refractory period prevents opening of voltage-gated Na+ channels.(C)the nodes of Ranvier conduct only in one direction.(D) the axon hillock has a higher membrane potential than the tips of the axon.(E) both sodium and potassium vollage-gated channels open in one direction.