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
Muscular rigidity caused by opioids is due to the agonistic effect on which receptor ? 
Select and explain. 
A. Mu, B. Kappa, C. Delta, D. Sigma
A: Mu
Answer: A: Mu 
Explanation: None