I think it is B, because that is my best guess!
The answer is that <span>healthy </span>legs<span> have </span>veins<span> with smooth, elastic walls that </span>are<span> perfectly designed to </span>adapt<span> to the changes in pressure within a </span>vein<span>. </span>Veins<span> have </span>valves<span> that keep blood moving in one direction: back toward the heart. As the </span>leg<span> muscles </span>are<span> activated, the </span>venous valves<span> open to </span>allow<span> one-way flow in the direction of the heart.</span>
