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': 'Stages of anaesthesia were established by – \nSelect and explain. \nA. Ether, B. N2O, C. Halothane, D. Chloroform', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Stages of anaesthesia were described by Guedel for ether.', 'role': 'assistant'}]