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
Choking off mechanism takes place after topical application of 
Select and explain. 
A. NaF, B. SnF2, C. APF, D. Sodium monofluorophosphate
A: NaF
Answer: A: NaF 
Explanation: None