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
[{'content': 'Recommended concentration of flouride in school water flouridation: \nSelect and explain. \nA. 4.5-6.3 PPm, B. 0.7-1.2PPm, C. 2.5-3.5PPm, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]