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': 'Amalgam contributes to etiology of chronic periodontal disease overlong because it acts as: \nSelect and explain. \nA. A mechanical irritant, B. Plaque retaining area, C. Nondeficiency tooth surface, D. overcontoured tooth surface', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]