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
Which index would you use to assess the severity of periodontitis in epidemiological studies of a large population? 
Select and explain. 
A. PMA index, B. Gingival index, C. Periodontal index, D. Sulcus bleeding index
C: Periodontal index
Answer: C: Periodontal index 
Explanation: None