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 of the following is a periodontal pathogen 
Select and explain. 
A. S. sanguis, B. S. mutans, C. A. viscosus, D. P. gingivalis
D: P. gingivalis
Answer: D: P. gingivalis 
Explanation: None