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
Periodontal index is also known as: 
Select and explain. 
A. Loe & Sillness index, B. Russel index, C. Ramford index, D. Shick & ash index
B: Russel index
Answer: B: Russel index 
Explanation: None