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': 'Which drug is used for pain control in cancer patients ? \nSelect and explain. \nA. Pethidine, B. Fentanyl, C. Methadone, D. Remifentanil', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]