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': 'All are opioid agonist - antagonist compounds except \nSelect and explain. \nA. Buprenorphine, B. Nalbuphine, C. Pentazocine, D. Nalmefene', 'role': 'user'}, {'content': 'Answer: D \nExplanation: D i.e. Nalmefene', 'role': 'assistant'}]