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 phenomenon is associated with 
Select and explain. 
A. APF gel, B. Stannous Fluoride, C. Potassium nitrate, D. Sodium Fluoride
D: Sodium Fluoride
Answer: D: Sodium Fluoride 
Explanation: 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.