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
Recommended concentration of flouride in school water flouridation: 
Select and explain. 
A. 4.5-6.3 PPm, B. 0.7-1.2PPm, C. 2.5-3.5PPm, D. None
A: 4.5-6.3 PPm
Answer: A: 4.5-6.3 PPm 
Explanation: None