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': 'Which of the following crystal form is least found in calculus: \nSelect and explain. \nA. Hydroxyapatite., B. Magnesium whitlockite., C. Octacalcium phosphate., D. Brushite.', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Hydroxyapatite 58%\nMagnesium whitlockite 21%\nOctacalcium phosphate\xa0 21%\nBrushite 9%', 'role': 'assistant'}]