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 of the following drugs does not possess even slightest agonist action ? 
Select and explain. 
A. Buprenorphine, B. Butorphanol, C. Nalbuphine, D. Nalmefene
D: Nalmefene
Answer: D: Nalmefene 
Explanation: None