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': 'How long does it take for incipient caries to develop into clinically evident smooth surface caries? \nSelect and explain. \nA. 9 ± 6 months, B. 2 + 6 months, C. 18 + 6 months, D. 24 ± 6 months', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]