Question

which blood vessels contain valves that prevent blood from flowing backwards on its way to the heart?
alveoli
arteries
veins
capillaries

Explanation:

Veins have valves to prevent back - flow of blood as blood in veins moves against gravity to return to the heart. Arteries carry blood away from the heart under high pressure and don't need such valves. Alveoli are for gas exchange, not blood flow control. Capillaries are for nutrient and gas exchange between blood and tissues.

Answer:

Veins