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: Anaesthesia
Which of the following opioids cause the least amount of nausea? 
Select and explain. 
A. Fentanyl, B. Sufentanil, C. Alfentanil, D. Remifentanil
C: Alfentanil
Answer: C: Alfentanil 
Explanation: Intraoperative use of opioids is a well-known risk factor for postoperative nausea and vomiting. Opioids stimulate the chemoreceptor trigger zone in the area postrema of the medulla, possibly through d-receptors, and thereby lead to nausea and vomiting. When compared with approximately equipotent doses of fentanyl and sufentanil, alfentanil is associated with a lower incidence of postoperative nausea and vomiting Ref: Miller's anesthesia-7th ed, Chapter 27.