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
True about slow sand filter is – 
Select and explain. 
A. Occupies less space, B. More expensive, C. Requires longer duration, D. Sand size 0.4 - 0.7 mm
C: Requires longer duration
Answer: C: Requires longer duration 
Explanation: Slow sand filter occupies large area (more space).
Slow and filter requires less skilled operation.
Slow sand filter removes more bacteria and has better bacterial control (99.9-99.99%)
Slow sand filter requires longer duration for purification.
Size of sand is smaller (0.2-0.3 mm).
Slow sand filter requires more time (duration).