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
Opioid agonist are all fo the following except 
Select and explain. 
A. Morphine, B. Codeine, C. Ketamine, D. Methadone
C: Ketamine
Answer: C: Ketamine 
Explanation: Ketamine