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
Variation in eruption in permanent dentition 
Select and explain. 
A. 4-5 months, B. 7-9 months, C. 1-2 months, D. 12-18 months
D: 12-18 months
Answer: D: 12-18 months 
Explanation: None