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: Psychiatry
Which drug is most commonly used world wide in maintenance treatment for opioid dependence: 
Select and explain. 
A. Naltrexone, B. Methadone, C. Imipramine, D. Disulfiram
B: Methadone
Answer: B: Methadone 
Explanation: B i.e. Methadone