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': 'The best sustaining topical effect of fluoride is obtained by: \nSelect and explain. \nA. NaF, B. APF gel, C. Duraphat, D. SnF', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Duraphat, the first fluoride varnish developed in Germany, is a viscous yellowish material, containing 22,600 ppm fluoride as sodium fluoride in a neutral colophonium base (NaF varnish containing 2.26% fluoride in organic\xa0lacquer)\n\nNaF- 9200 ppm\nSnF2\xa0- 9500 ppm\nAPF- 12300 ppm\n\nEssentials of preventive and community dentistry 5th edition\nSoben Peter page no. 256', 'role': 'assistant'}]