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': '0.02% NaF means? \nSelect and explain. \nA. 90 ppm, B. 900 ppm, C. 225 ppm, D. 500 ppm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]