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': 'Gas cylinder with single pin index ? \nSelect and explain. \nA. Oxygen, B. Air, C. Nitrogen, D. Entonox', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is'd' i.e., Entonox", 'role': 'assistant'}]