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 they carry blood back to the heart under low pressure. Alveoli are for gas exchange in the lungs, arteries carry blood away from the heart and don't have these types of valves, and capillaries are for exchange of substances between blood and tissues.

Answer:

C. Veins