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
Feature of slow filter (w.r.t. fast filter) is/are: 
Select and explain. 
A. Occupies less space, B. Highly skilled operation, C. Poor bacterial quality, D. Takes more time for purification
D: Takes more time for purification
Answer: D: Takes more time for purification 
Explanation: Ans. is 'd' i.e. Takes more time for purification o Slow sand filter occupies large area (more space). o Slow and filter requires less skilled operation. o Slow sand filter removes more bacteria and has better bacterial control (99.9-99.99%) o Slow sand filter requires longer duration for purification. o Size of sand is smaller (0.2-0.3 mm).