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: Anaesthesia
[{'content': 'The concentration of O2 in adequate anaesthesia is – \nSelect and explain. \nA. 15%, B. 18%, C. 33%, D. 50%', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]