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
Which of these is not an opioid agonist: AIIMS 10 
Select and explain. 
A. Heroin, B. Ketamine, C. Methadone, D. Fentanyl
B: Ketamine
Answer: B: Ketamine 
Explanation: Ans. Ketamine