Answer : 3. "their walls are only one cell thick"
Explanation :

Answer: C) their walls are only one cell thick Explanation: Capillaries Tiny blood vessels connecting arteries to veins. These blood vessels carry oxygen and nutrients to individual cells. Capillaries are the smallest blood vessels in the body, and have walls only one or two cells thick. (Their diameter is 5 to 10 micrometers, or 5 to 10 millionths of a meter.)   Capillaries connect arterioles and venules, which are the smaller extensions of the larger arteries and veins.