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
The drug for OPD analgesia is : 
Select and explain. 
A. Morphine, B. Pethidine, C. Fentanyl, D. Alfentanil
D: Alfentanil
Answer: D: Alfentanil 
Explanation: None