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
0.02% NaF means? 
Select and explain. 
A. 90 ppm, B. 900 ppm, C. 225 ppm, D. 500 ppm
C: 225 ppm
Answer: C: 225 ppm 
Explanation: None