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
Longest acting β–agonist is : 
Select and explain. 
A. Salbutamol, B. Terbutaline, C. Salmeterol, D. Theophylline
C: Salmeterol
Answer: C: Salmeterol 
Explanation: None