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: Physiology
Which of the following is an endothelial smooth muscle relaxing factor? 
Select and explain. 
A. Nitric oxide, B. Angiotensin, C. Dopamine, D. Vasopressin
A: Nitric oxide
Answer: A: Nitric oxide 
Explanation: Nitric oxide acts in paracrine manner on target cells through induction of cGMP which in turn initiates a series of intravascular events leading to relaxation of vascular smooth muscle.