The Opening and Closing of the Heart Valves Is Caused

The opening and closing of the heart valves is caused by

A) contraction and relaxation of papillary muscles that pull on heart strings.
B) pressure changes of alternating contraction and relaxation during the cardiac cycle.
C) contraction of the smooth muscle in the walls of the great vessels leaving the heart.
D) action potentials within the cusps of the valves.