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': 'Following scaling a patient notices hard black deposits. This may be due to: \nSelect and explain. \nA. Shrinkage of gingiva, B. Patient is heavy tea drinker, C. Poor home care, D. Blood clot formation on tooth surface', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]