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
Which of the following is least narcotic opioid ? 
Select and explain. 
A. Morphine, B. Codeine, C. Heroin, D. Papaverine
D: Papaverine
Answer: D: Papaverine 
Explanation: None