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 among the following is a pure antagonist of opioid receptors? 
Select and explain. 
A. Naltrexone, B. Nalbuphine, C. Butorphanol, D. Pentazocine
A: Naltrexone
Answer: A: Naltrexone 
Explanation: None