All of the following are classification of blood vessels except: A. Valve B. Arteries C. Capillaries D. Veins

The correct answer is **A. Valve**.<br /><br />Arteries, capillaries, and veins are all types of blood vessels. Valves are structures *within* veins (and the heart) that prevent backflow of blood, but they are not a classification of blood vessel themselves.<br />