Blood Moves into and Then Out of a Heart Chamber

Multiple Choice

Blood moves into and then out of a heart chamber because

A) it moves along its pressure gradient, and that gradient depends on contraction and relaxation during the cardiac cycle.
B) it is under constant pressure, but its movement is dictated by the control of valve openings and closures.
C) the veins and arteries constrict and dilate to propel and attract blood.
D) All of the choices are correct.