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
Pure opiate antagonists are all of the following except: 
Select and explain. 
A. Naloxone, B. Nalorphine, C. Nalmefene, D. Naltrexone
B: Nalorphine
Answer: B: Nalorphine 
Explanation: None