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': 'Gas used to measure diffusion in lung: \nSelect and explain. \nA. CO, B. NO, C. CO2, D. Nitrogen', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. CO', 'role': 'assistant'}]