What problem may occur if veins do not have valves?

Correct answer: Blood may flow backward

Veins return blood to the heart at relatively low pressure, especially from the lower parts of the body. Valves open toward the heart and close when blood tends to reverse direction. Without these one-way valves, gravity and muscle relaxation could allow pooling or backward flow, making venous return less efficient. Therefore option A is correct; the other options confuse circulation with unrelated processes.