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
Which of the following crystal form is least found in calculus: 
Select and explain. 
A. Hydroxyapatite., B. Magnesium whitlockite., C. Octacalcium phosphate., D. Brushite.
D: Brushite.
Answer: D: Brushite. 
Explanation: Hydroxyapatite 58%
Magnesium whitlockite 21%
Octacalcium phosphate  21%
Brushite 9%