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 with greatest affinity for haemoglobin \nSelect and explain. \nA. O2, B. CO, C. CO2, D. N2O', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]