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': 'NaF application is done: \nSelect and explain. \nA. Once a year, B. twice a year, C. four times a year, D. eight times a year', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]