The plasma membrane of a neuron has voltage-gated sodium and potassium channels. What is the effect ...

The plasma membrane of a neuron has voltage-gated sodium and potassium channels. What is the effect of membrane depolarization on these channels?

a) Membrane depolarization opens sodium channels but closes potassium channels.
b) Membrane depolarization opens sodium and potassium channels at the same time.
c) Membrane depolarization first opens sodium channels and then opens potassium channels

Why is an action potential an all-or-none response to stimuli?
a) Because a typical neuron receives signals through multiple dendrites but transmits signals through a single axon
b) Because neurons contain gated ion channels that are either open or closed
c) Because voltage-gated ion channels open when membrane potential passes a particular level