This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Local and Regional Anesthesia, Subject name: Anaesthesia
[{'content': 'Pin index system for nitrous oxide: \nSelect and explain. \nA. 1,6, B. 2,5, C. 3,5, D. 1,5', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans: (c) 3,5Ref: Miller's Anaesthesia 7th ed. / 675GasColor CodePin index positionBodyShoulderOxygenBlackWhite2,5Nitrous oxideBlueBlue3, 5CO2GrayGray1,6HeliumBrownBrown2,5AirGreyWhite/black quartered1,5", 'role': 'assistant'}]