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': 'The following drug is a selective blocker (antagonist) of  M1 muscarinic receptors: \nSelect and explain. \nA. Methacholine, B. Bethanechol, C. Methoctramine, D. Pirenzepine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]