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: Physiology
Which of the following relaxes bronchial smooth muscles? 
Select and explain. 
A. Cold air, B. Leukotriene, C. Ach, D. VIP
D: VIP
Answer: D: VIP 
Explanation: Introduction. A major symptom of asthma is an excessive contraction of airway smooth muscle cells (SMCs) which results in airway hyper-reactivity. To allete this acute and chronic airway constriction, b-adrenergic agonists, that relax SMCs, are commonly administered Ref: guyton and hall textbook of medical physiology 12 edition page number:331,332,333