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
A 6 year old child does not have any permanent tooth erupted in oral cavity. By what maximum age the tooth should erupt normally? 
Select and explain. 
A. 7.5 years, B. 8.5 years, C. 6.5 years, D. all of the above
A: 7.5 years
Answer: A: 7.5 years 
Explanation: None