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Option a. is incorrect. Veins are blood vessels that return blood to the heart. Option c. is incorrect. Arteries are blood vessels that transport blood away from the heart. Option d. is incorrect. The atria are the upper chambers of the heart that receive blood from the body. Option b. is incorrect. Capillaries are the smallest blood vessels in the body. These vessels have the diameter of approximately a single red blood cell. It is through the thin walls of capillaries that nutrients, wastes, and gases are exchanged.