Why do capillaries need to be thin walled?

CAPILLARIES are the smallest blood vessels in the body.

A single capillary is so small that it allows only one blood cell to flow through it at a time. The capillary walls are also very small, only one cell thick. These thin walls easily allow water, oxygen, carbon dioxide, and other nutrient and waste substances to exchange between blood cells and the surrounding tissue.

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the answer would be..

b. sucking