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': 'In topical fluoride application, maximum benefit is\nderived when the tooth surface is treated for: \nSelect and explain. \nA. 15 seconds, B. 30 seconds, C. 1 minute, D. 4 minutes', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]