This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
Which of the following drug is not an opioid agonist? 
Select and explain. 
A. Heroin, B. Ketamine, C. Methadone, D. Codiene
B: Ketamine
Answer: B: Ketamine 
Explanation: Heroin is a semi synthetic opiod agonists . Codiene is a natural opium alkaloid . Methadone is a synthetic alkaloid Ref": KD Tripathi 8 th ed.