This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
[{'content': 'Which of the following for pain control in terminally ill cancer patients \nSelect and explain. \nA. Pethidine, B. Fentanyl, C. Methodone, D. Remifentanyl', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Fentanyl, also spelled fentanil, is an opioid used as a pain medication and together with other medications for anesthesia. Fentanyl is also used as a recreational drug, often mixed with heroin or cocaine. It has a rapid onset and effects generally last less than two hours. Ref: KD TRIPATHI 8TH ED.', 'role': 'assistant'}]