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
Type-2 pulmonary epithelial cells secrete 
Select and explain. 
A. Surfactant, B. Mucus, C. Heparin, D. Polypeptides
A: Surfactant
Answer: A: Surfactant 
Explanation: A