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': 'Root completion of the mandibular first molar takes place by the age: \nSelect and explain. \nA. 7-8 years, B. 8-9 years, C. 9-10 years, D. 10-11 years', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]