When the threshold level is reached and depolarization occurs, how come only the sodium ions begin ...

Both potassium and sodium channels located along the nerve membrane are voltaged gated. This means that they respond to the changes in the voltage, by opening and closing the gates to allow the ions to flow in or out of the membrane.

When the threshold level is reached and depolarization occurs, how come only the sodium ions begin to rush in?
Select one:
a. The threshold level for both sodium and potassium channels are equal but sodium channels slower to open.
b. The threshold level for both sodium and potassium channels are equal but potassium channels take longer to open.
c. The threshold level for the potassium channels are lower than the sodium channels.
d. The threshold level for the potassium channels are higher than the sodium channels.