This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Pharmacology
Wof b2 agonist bronchodilators is given by inhalation, and is suitable for both terminating asthma attacks as well as for twice daily prophylaxis: 
Select and explain. 
A. Terbutaline, B. Bambuterol, C. Salmeterol, D. Formoterol
D: Formoterol
Answer: D: Formoterol 
Explanation: Only long acting drugs can be used as BD dose. Formoterol:- A long acting selective b2 agonist which acts for 12 hrs when inhaled(can be used twice a day). Compared to salmeterol it has a faster onset of action. It is used on a regular morning-evening schedule for round-the-clock bronchodilatation. Salmeterol:- It is long acting selective b2 agonist with a slow onset of action; used by inhalation on a twice daily schedule for maintenance therapy and for nocturnal asthma, but not for acute symptoms. Ref:- kd tripathi; pg num:-223,224