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': 'Mouth-to-mouth respiration provides an oxygen concentration of \nSelect and explain. \nA. 16%, B. 20%, C. 22%, D. 24%', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The concentration of O2 in the expired air is 16%.', 'role': 'assistant'}]