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 phenomenon is associated with \nSelect and explain. \nA. APF gel, B. Stannous Fluoride, C. Potassium nitrate, D. Sodium Fluoride', 'role': 'user'}, {'content': 'Answer: D \nExplanation: When sodium fluoride is applied topically it reacts with hydroxyappetite crystals to form calcium fluoride. Once a thick layer of calcium fluoride is formed, it interferes with the further diffusion of fluoride.', 'role': 'assistant'}]