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': 'Which index would you use to assess the severity of periodontitis in epidemiological studies of a large population? \nSelect and explain. \nA. PMA index, B. Gingival index, C. Periodontal index, D. Sulcus bleeding index', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]