Answer: C - Within the capillaries, oxygen, and fuel are exchanged for waste products. The diameter of a capillary is small, requiring blood cells to squeeze through in single file. Capillary walls are thin; they are only one cell thick. The thin walls and single-file flow allow exchanges to take place. Arterioles feed oxygenated blood into capillary vessels; venules receive the deoxygenated blood and begin the return flow to the heart.