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
In a slow sand filter, the element responsible for yielding bacteria-free water is the - 
Select and explain. 
A. Valve, B. Vital layer, C. Supernatant water, D. Under-drainage system
B: Vital layer
Answer: B: Vital layer 
Explanation: Ans. is 'b' i.e., Vital layer