During the S-T Segment, What Is Occurring Within Cardiac Muscle

Multiple Choice

During the S-T segment, what is occurring within cardiac muscle cells?

A) Sodium is rapidly diffusing out of atrial muscle cells.
B) Sodium channels are beginning to open in ventricular cells and calcium is entering through slow channels in atrial cells.
C) Calcium is entering and potassium is leaving ventricular cells.
D) Potassium is entering atrial cells and sodium is leaving ventricular cells.