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
Which of the following is 100 times more potent than morphine? 
Select and explain. 
A. Pethidine, B. Fentanyl, C. Pentazocin, D. Meperidine
B: Fentanyl
Answer: B: Fentanyl 
Explanation: Fentanyl is 100 times more potent than morphine. Pethidine is 1/10th as potent as morphine. Ref KD Tripathi 8th ed.