The "undershoot" phase of after-hyperpolarization is ...

[geoffrey]

The "undershoot" phase of after-hyperpolarization is due to
A) slow opening of voltage-gated sodium channels.
B) sustained opening of voltage-gated potassium channels.
C) rapid opening of voltage-gated calcium channels.
D) slow restorative actions of the sodium-potassium ATPase.
E) ions that move away from their open ion channels.

[j_sun]

I recall doing this last year, it's B, bye bye!