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
All of the following are common reason for periodontitis except: 
Select and explain. 
A. Increases in HLAA9, B. Increase in B15, C. Decrease in HLAA2, D. Increase in HLAA2
D: Increase in HLAA2
Answer: D: Increase in HLAA2 
Explanation: None