Which of the Following Is Responsible for the Falling Phase

Multiple Choice

Which of the following is responsible for the falling phase of an action potential?

A) opening of Na⁺ gates
B) Na⁺-K⁺ pump restoring the ions to their original locations
C) greatly increased permeability to K⁺
D) ATP-ase destroying the energy supply that was maintaining the action potential at its peak.
E) none of these answers.