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': 'Recent classification of periodontal disease organized by American Academy of Periodontology was introduced in the year: \nSelect and explain. \nA. 1989, B. 1997, C. 1998, D. 1999', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]