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': 'Dunning index? \nSelect and explain. \nA. Gingival bone count, B. Shick & ash, C. OHI, D. PDI', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]