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': "Method of application of neutral sodium fluoride according to Knutson's technique involves all except- \nSelect and explain. \nA. At the initial appointment, the teeth are first cleaned with aqueous pumice slurry and then isolated with cotton rolls and dried with compressed  air., B. The sodium fluoride solution applied on the teeth with cotton-tipped applicator stick is allowed to dry for 3 to 4 minutes., C. A  second, third  and  fourth fluoride application, each preceded  by a prophylaxis, is scheduled at intervals of approximately one week., D. The four-visit procedure is recommended for ages 3, 7, 1 1 and 13 years.", 'role': 'user'}, {'content': "Answer: C \nExplanation: Method of application of neutral sodium fluoride according to Knutson's technique-\nIf the sodium fluoride reagent is pure and uncontaminated, this solution has a pH of 7. Treatments are given in a series of four appointments.\xa0\n\nAt the initial appointment, the teeth are first cleaned with aqueous pumice slurry and then isolated with cotton rolls and\xa0dried with compressed\xa0 air. Teeth can either be isolated by quadrant or by half mouth.\xa0\nUsing cotton-tipped applicator sticks, the 2% sodium fluoride solution is painted on the air-dried teeth so that all surfaces are\xa0visibly wet. Then\xa0 with the teeth still isolated, the solution is allowed to dry for 3 to 4 minutes.\xa0\nThis procedure is repeated for each of the isolated segments until all of the teeth are treated.\xa0\nA\xa0 second, third\xa0 and\xa0 fourth fluoride application, each\xa0 not preceded\xa0 by a prophylaxis, is scheduled at intervals of\xa0approximately one week.\xa0\nThe four-visit procedure is recommended for ages 3, 7, 11 and 13 years, coinciding with the eruption of different groups of\xa0primary and permanent teeth. Thus, most of the teeth would be treated soon after their eruption, maximizing the protection\xa0afforded by topical application.", 'role': 'assistant'}]