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: Pharmacology
[{'content': 'Opioid agonist are all fo the following except \nSelect and explain. \nA. Morphine, B. Codeine, C. Ketamine, D. Methadone', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ketamine', 'role': 'assistant'}]