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
[{'content': 'Muscular rigidity caused by opioids is due to the agonistic effect on which receptor ? \nSelect and explain. \nA. Mu, B. Kappa, C. Delta, D. Sigma', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]