In Humans, Approximately How Much of the Blood Volume Lies

In humans, approximately how much of the blood volume lies in the veins, and how do veins raise blood back to the heart?

A) 65% of blood is located in veins, which raise blood to the heart by blood pressure alone.
B) 50% of blood is located in veins, which raise blood to the heart via a diffusible gradient.
C) 75% of blood is located in veins, which raise blood to the heart by compression of muscles located in the vein walls.
D) 40% of blood is located in veins, which raise blood to the heart by gravity.
E) 75% of blood is located in veins, which raise blood to the heart by pressure and external compression resulting from contraction of adjacent muscles.