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': 'In periodontal disease, calculus is considered as \nSelect and explain. \nA. Contributing factor, B. Primary factor, C. Etiological factor, D. Governing factor', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]