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
Feature of slow filter (w.r.t. Faster filter) is/are: 
Select and explain. 
A. Occupies less space, B. Highly skilled operation, C. Poor bacterial quality, D. Takes more time purification
D: Takes more time purification
Answer: D: Takes more time purification 
Explanation: Slow sand filters occupy more space, require less skilled operators and gives good bacterial quality when compared to rapid sand filters. But it takes more time for the operation. Rate of filtration is 200 m.g.a.d in the rapid sand filter but only 2-3m.g.a.d. in the slow sand filter (PARK TB 25th edition pg 773)