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
How long does it take for incipient caries to develop into clinically evident smooth surface caries? 
Select and explain. 
A. 9 ± 6 months, B. 2 + 6 months, C. 18 + 6 months, D. 24 ± 6 months
C: 18 + 6 months
Answer: C: 18 + 6 months 
Explanation: None