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': 'Organic content of supragingival calculs: \nSelect and explain. \nA. Hydroxyapatite crystal + octacalcium phosphate., B. Hydroxyapatite crystal + Magnesium whit lockite., C. Hydroxyapatite crystal +Brushite., D. Hydroxyapatite crystal + Calcium phosphate.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]