Question

have valves to prevent backflow.

Explanation:

Veins are blood vessels that carry blood back to the heart. Since the blood flow in veins is against gravity in some parts of the body (e.g., legs), valves are present to prevent the backflow of blood. Arteries, which carry blood away from the heart, do not have these valves as the pressure from the heart's pumping action is sufficient to keep the blood moving forward.

Answer:

Veins