This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anesthesia Circuit, Subject name: Anaesthesia
[{'content': 'Which one of the circuits does not have valves? \nSelect and explain. \nA. Magills circuit, B. Bains circuit, C. Closed circuit, D. Mapleson E', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ayre's T-piece was invented by Phillip Ayer in 1937. This consists of a light metal tube 1 cm in diameter, 5 cm in length with a side arm. Used as such, it functions as a non-rebreathing system. Fresh gas enters the system through the side arm and the expired gas is vented into the atmosphere and there is no rebreathing.", 'role': 'assistant'}]