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: None