The "Lub" of the Lub-DUB Sounds Heard Through a Stethoscope

The "lub" of the lub-DUB sounds heard through a stethoscope when listening to the heart is produced by the

A) inferior vena cava and superior vena cava emptying into the right atrium.
B) closing of the two atrioventricular valves.
C) contraction of the atria.
D) closing of the aortic valve and pulmonary valve.
E) pulmonary arteries emptying into the left atrium.