What Happens During Phase 4 of the Cardiac Action Potential

What happens during Phase 4 of the cardiac action potential?

A) Potassium ions efflux out of the cardiac membrane as it undergoes repolarization.
B) Sodium ions enter the cardiac membrane as it undergoes depolarization.
C) The membrane potential returns to its resting level and prepares itself for the next action potential.
D) Calcium channels open and calcium ions pass into muscle cells.