One cell-thick vessels are called I. Arteries II. Veins III. Capillaries IV. Pulmonary artery
The correct option is A. Explanation for the correct option: One cell thick vessels are the blood vessels that join arteries and veins are the tiniest and most abundant blood vessels, called capillaries. They primarily serve the purpose of exchanging materials between the blood and tissue cells. By releasing the carbon dioxide, it serves as a center of circulation where the oxygen is delivered to the tissue.
