What is the role of valves in the veins?

A) They push the blood back toward the capillaries so that all the oxygen can be removed.
B) They push the blood upward toward the heart.
C) They slow blood flow in the veins so that the blood can move more slowly in the capillaries.
D) They block the flow of blood to the heart until it has time to contract again.
E) They keep blood from flowing backward when the veins are squeezed by skeletal muscles.