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: Social & Preventive Medicine
"Vital Layer" in slow sand filter is: 
Select and explain. 
A. Sand bed, B. Zoological layer, C. Supernatan, D. Under drainage layer
B: Zoological layer
Answer: B: Zoological layer 
Explanation: Zoological layer