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
Main crystalline component of calculus is 
Select and explain. 
A. Hydroxyapatite, B. Octacalcium phosphate, C. Magnisium whitlockite, D. Brushite
A: Hydroxyapatite
Answer: A: Hydroxyapatite 
Explanation: Hydroxyapatite and octacalcium phosphate(constitute the bulk of the specimen) are detected most frequently in all supragingival calculus.
While in Subgingival calculus ratio of calcium to phosphate is higher subgingivally and sodium content also increases with the depth of periodontal pocket.