One-cell thick blood vessels are known as:

The chapter states: "The smallest vessels have walls which are one-cell thick and are called capillaries. Exchange of material between the blood and surrounding cells takes place across this thin wall." Hence the correct answer is capillaries (B). Option A, alveoli, are balloon-like air sacs in the lungs where gas exchange occurs across their walls (which do have a rich network of blood capillaries), but alveoli themselves are not blood vessels, so this option is incorrect. Option C, arteries, are described as vessels that "carry blood away from the heart" and have "thick, elastic walls" because blood emerges under high pressure — the opposite of one-cell-thick. Option D, veins, "collect the blood from different organs and bring it back to the heart" and, while they do not need thick walls since blood is not under pressure, the text specifically reserves the one-cell-thick description for capillaries, not veins, which instead have valves to prevent backflow.