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': 'All of the following are common reason for periodontitis except: \nSelect and explain. \nA. Increases in HLAA9, B. Increase in B15, C. Decrease in HLAA2, D. Increase in HLAA2', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]