Answer the following question given this paragraph: 

Veins are blood vessels that carry blood toward the heart. This blood is no longer under pressure, so veins have thinner walls. To keep the blood moving, many veins have valves that prevent the backflow of blood. Veins generally carry oxygen-poor blood. The smallest veins are called venules. They merge to form larger and larger veins. The largest vein is the inferior vena cava, which carries blood from the lower body directly to the heart.


Q: What are the blood vessels that carry blood toward the heart and tend to have thinner walls?

 Choices:

- capillaries

- glands

- clots

- veins

A:
veins
