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
What is the size of paicle is rapid sand filter? 
Select and explain. 
A. 0.1 mm, B. 0.2, C. 0.5, D. 0.4 to 0.7 mm
D: 0.4 to 0.7 mm
Answer: D: 0.4 to 0.7 mm 
Explanation: The effective size of the sand paicle is 0.4-0.7 mm in case of the rapid sand filter and 0.2-0.3 mm in case of the slow sand filter. (PARK TB 25th edition pg 774)