(C) Veins generally contain valves to prevent the backward flow of blood. However,the pulmonary vein is an exception as it lacks valves. This is becau

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(C) Veins generally contain valves to prevent the backward flow of blood. However,the pulmonary vein is an exception as it lacks valves. This is because the opening of the pulmonary vein into the left atrium is oblique,which naturally prevents the backflow of blood when the atrium contracts.