Cardiac Tetany Never Occurs Under Normal Conditions for Which of These

Cardiac tetany never occurs under normal conditions for which of these reasons

A) because there are no distinct motor units in the heart
B) because there is inadequate oxygen supply via the coronary circulation
C) because of the long duration of cardiac muscle's refractory period preventing summation of contraction
D) because the heart contracts with maximal force every beat, increasing the strength of the cardiac contraction