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
[{'content': 'Nolla stage 6 means: \nSelect and explain. \nA. Crown completion, B. 1/3rd root completed, C. 2/3rd root completed, D. Crown almost completed', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]