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: Dental
21 Cities Study by Dean is done for? 
Select and explain. 
A. Checking hypothesis of 1 ppm is minimum threshold in water than not leads to fluorosis, B. Check first fluoridated water supply city in USA, C. Check severity of mottling enamel in different regions of USA, D. Checking best city with least caries % of affected tooth in 0-5 years of children
A: Checking hypothesis of 1 ppm is minimum threshold in water than not leads to fluorosis
Answer: A: Checking hypothesis of 1 ppm is minimum threshold in water than not leads to fluorosis 
Explanation: None