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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Veins are blood vessels that carry blood toward the heart. This blood is no longer under much pressure, so many veins have valves that prevent backflow of blood. Veins generally carry deoxygenated blood. The largest vein is the inferior vena cava, which carries blood from the lower body to the heart. Answer: heart
Output:
Veins are blood vessels that carry blood toward what?