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
The following drug is a selective blocker (antagonist) of  M1 muscarinic receptors: 
Select and explain. 
A. Methacholine, B. Bethanechol, C. Methoctramine, D. Pirenzepine
D: Pirenzepine
Answer: D: Pirenzepine 
Explanation: None