Which of the Following Statements Best Describes the Mechanism(s)by Which

Which of the following statements best describes the mechanism(s) by which venous blood returns to the heart?

A) Intraluminal valves ensure unidirectional flow toward the heart.
B) Contracting skeletal muscles milk blood distally toward the veins.
C) The high-pressure system of the heart helps to facilitate venous return.
D) Increased thoracic pressure and decreased abdominal pressure facilitate venous return to the heart.