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
Nolla stage 6 means: 
Select and explain. 
A. Crown completion, B. 1/3rd root completed, C. 2/3rd root completed, D. Crown almost completed
A: Crown completion
Answer: A: Crown completion 
Explanation: None