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
The common etiology of periodontitis is 
Select and explain. 
A. Occlusal trauma, B. Systemic factors, C. Local irritating factors, D. Hormonal defects
C: Local irritating factors
Answer: C: Local irritating factors 
Explanation: None