When You Are Upright, What Is the Primary Mechanism That

When you are upright, what is the primary mechanism that propels blood in your feet back to your inferior vena cava?

A) recoil of elastic fibers in the walls of the leg veins
B) contraction of the left ventricle of the heart
C) contraction of smooth muscle in the walls of the leg veins
D) contraction of skeletal muscles in your legs