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
In how many years, the mandibular central incisors erupt as permanent teeth? 
Select and explain. 
A. 8-9 years, B. 10-12 years, C. 6-7 years, D. 17-25 years
C: 6-7 years
Answer: C: 6-7 years 
Explanation: None