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: Forensic Medicine
The drug which is used for long-term maintenance in opioid addiction: AIIMS 06; Punjab 09; AI 11 
Select and explain. 
A. Nalorphine, B. Naloxone, C. Butarphanol, D. Methadone
D: Methadone
Answer: D: Methadone 
Explanation: Ans. Methadone