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
Recent classification of periodontal disease organized by American Academy of Periodontology was introduced in the year: 
Select and explain. 
A. 1989, B. 1997, C. 1998, D. 1999
D: 1999
Answer: D: 1999 
Explanation: None