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': 'Prepubertal periodontitis occurs \nSelect and explain. \nA. Before 11 yrs, B. At 12-15 years, C. At 18 years of age, D. After 20 years', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]