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: Surgery
[{'content': 'All post-operative patients must be supplemented with\noxygen concentration of at least\n: \nSelect and explain. \nA. 50-60%, B. 40-45%, C. 30-35%, D. 20-25%', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]