This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Vital layer in slow sand filter is seen in - 
Select and explain. 
A. Top of water, B. On the sand bed, C. Near filter, D. None
B: On the sand bed
Answer: B: On the sand bed 
Explanation: Elements of slow sand filter: supernatant or raw water (1-1.5 metre) a bed of graded sand an under-drainage system a system of filter control valves The surface of the sand bed gets covered with a slimy growth known as "Schmutzdecke", vital layer, zoological or biological layer. The formation of the vital layer is called ripening. It is the hea of the slow sand filter Park's textbook of preventive and social medicine.K Park. Edition 25. page no:772