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
Organic content of supragingival calculs: 
Select and explain. 
A. Hydroxyapatite crystal + octacalcium phosphate., B. Hydroxyapatite crystal + Magnesium whit lockite., C. Hydroxyapatite crystal +Brushite., D. Hydroxyapatite crystal + Calcium phosphate.
A: Hydroxyapatite crystal + octacalcium phosphate.
Answer: A: Hydroxyapatite crystal + octacalcium phosphate. 
Explanation: None