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: Dental
[{'content': 'The minimal concentration of oxygen that should be used with Nitrous oxide analgesia is: \nSelect and explain. \nA. 10%, B. 20%, C. 30%, D. 40%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Nitrous oxide is a weak anaesthetic and while some patients can be brought to an unconscious level by mixture of Nitrous oxide and Oxygen containing at least 20% Oxygen.', 'role': 'assistant'}]