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
Surfactant is: 
Select and explain. 
A. Protein, B. Glycoprotein, C. Sphingolipid, D. Alpha palmitoyl lecithin
D: Alpha palmitoyl lecithin
Answer: D: Alpha palmitoyl lecithin 
Explanation: D