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
[{'content': 'Which among the following is a pure antagonist of opioid receptors? \nSelect and explain. \nA. Naltrexone, B. Nalbuphine, C. Butorphanol, D. Pentazocine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]