The best surface to run on?
the answer is b: smooth and soft so that it can cushion the stresses of running
They can constrict or dilate because their walls are muscular.
The second statement best explains the type of blood vessel and the function of these vessels' valves - <span>These are veins, which prevent the backflow of blood because the walls have little muscle to pump blood back to the heart.
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Veins are large blood vessels which carry blood back to the heart. <span>Veins have a system of valves to prevent back-flow.</span>
