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
[{'content': 'Which of the following vessels have the least diameter in cross section \nSelect and explain. \nA. Veins, B. Arteriole, C. Venule, D. Capillaries', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The capillaries are about 5 μm in diameter at the arterial end and 9 μm in diameter at the venous end.', 'role': 'assistant'}]