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 the sand bed, B. Top of water, C. Near filter valves, D. None
A: On the sand bed
Answer: A: On the sand bed 
Explanation: None