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
Effective size of sand in rapid sand filter is 
Select and explain. 
A. 0.2 mm, B. 0.1 mm, C. 0.8 mm, D. 0.5 mm
D: 0.5 mm
Answer: D: 0.5 mm 
Explanation: None