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': 'When the concentration of the etchant is more than 50%, it forms a complex which prevents further dissolution. The complex is \nSelect and explain. \nA. hydroxyapatite, B. calcium phosphate, C. monocalcium phosphate monohydrate, D. none of the above', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]