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: Anaesthesia
All are opioid agonist - antagonist compounds except 
Select and explain. 
A. Buprenorphine, B. Nalbuphine, C. Pentazocine, D. Nalmefene
D: Nalmefene
Answer: D: Nalmefene 
Explanation: D i.e. Nalmefene