The Body Has Several Ways to Keep Blood Moving Forward

The body has several ways to keep blood moving forward through the veins.Which of the following does NOT explain how this happens?

A) The pressure in the veins is lower than the pressure in the arteries (i.e.blood flows down its pressure gradient) .
B) Valves in the veins prevent backflow.
C) The aid of respiratory pump (changes in pressure in thoracic cavity during inhalation and exhalation) .
D) The pumping of the venous endothelial contraction nodules (which act like a series of miniature hearts) .
E) The aid of contractions of skeletal muscles.