This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following vessels have the least diameter in cross section 
Select and explain. 
A. Veins, B. Arteriole, C. Venule, D. Capillaries
D: Capillaries
Answer: D: Capillaries 
Explanation: The capillaries are about 5 μm in diameter at the arterial end and 9 μm in diameter at the venous end.