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': 'Transport of carbon monoxide (CO) is diffusion limited because \nSelect and explain. \nA. High affinity of CO for haemoglobin, B. Alveolar membrane is less permeable to CO, C. CO crosses epithelial barrier slowly, D. On exposure to air there is sudden increase in partial pressure', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Since CO binds to Hb very rapidly, there is always a pressure gradient for its diffusion. \nThere is no free CO in blood. CO diffusion can be increased only if the thickness of the capillary decreases or if the area of the capillary increases. \nSo transport of CO is diffusion limited.', 'role': 'assistant'}]