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
The vital layer in a slow sand filter is 
Select and explain. 
A. Sand bed, B. Under drainage, C. Zoological layer, D. Supernatant
C: Zoological layer
Answer: C: Zoological layer 
Explanation: None