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
When the concentration of the etchant is more than 50%, it forms a complex which prevents further dissolution. The complex is 
Select and explain. 
A. hydroxyapatite, B. calcium phosphate, C. monocalcium phosphate monohydrate, D. none of the above
C: monocalcium phosphate monohydrate
Answer: C: monocalcium phosphate monohydrate 
Explanation: None