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': 'Choking off mechanism takes place after topical application of \nSelect and explain. \nA. NaF, B. SnF2, C. APF, D. Sodium monofluorophosphate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]