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
Which drug is used for pain control in cancer patients ? 
Select and explain. 
A. Pethidine, B. Fentanyl, C. Methadone, D. Remifentanil
B: Fentanyl
Answer: B: Fentanyl 
Explanation: None