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 seen: 
Select and explain. 
A. On top of the water, B. On the sand bed, C. Just below the supernatant water, D. None of the above
B: On the sand bed
Answer: B: On the sand bed 
Explanation: When a sand filter is newly laid, it merely acts as a mechanical strainer. Soon the surface of sand bed is covered by a slimy growth known as vital layer consisting of algae, planktons, bacteria, diatoms etc. This vital layer holds back bacteria and oxidises ammoniacal nitrogen to nitrates and yields a bacteria free water. Ref: Park 21st edition page: 659.