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
Tramadol is: 
Select and explain. 
A. Antiflatulent, B. Antireflux drug, C. Beta–blocker, D. Opioid analgesic
D: Opioid analgesic
Answer: D: Opioid analgesic 
Explanation: None