The Following Diagram of a Nerve Action Potential Applies to Questions

The following diagram of a nerve action potential applies to Questions :

-What process is responsible for the change in membrane potential that occurs between point 1 and point 3 ?

A) Movement of $\mathrm{Na}^{+}$ into the cell
B) Movement of $\mathrm{Na}^{+}$ out of the cell
C) Movement of $\mathrm{K}^{+}$ into the cell
D) Movement of $\mathrm{K}^{+}$ out of the cell
E) Activation of the $\mathrm{Na}^{+}-\mathrm{K}^{+}$ pump
F) Inhibition of the $\mathrm{Na}^{+}-\mathrm{K}^{+}$ pump