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': 'Hallucination is the major side effect of : \nSelect and explain. \nA. Nitrous oxide., B. Ketamine, C. Fentanyl, D. Chloroform', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]