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': 'The common etiology of periodontitis is \nSelect and explain. \nA. Occlusal trauma, B. Systemic factors, C. Local irritating factors, D. Hormonal defects', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]