The Heartbeat Originates in The

The heartbeat originates in the:

A) sinoatrial node located at the junction of the vena cava and the right atrium.
B) sinoatrial node located at the junction of the vena cava and the left atrium.
C) atrioventricular node located in the wall of the atrioventricular valve.
D) atrioventricular node located in the tissue separating the atria and ventricles.
E) sinoatrial node and atrioventricular node simultaneously.