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
Enamel crystallites are stabilized against acid if fluoride ions replace 
Select and explain. 
A. Phosphate ions, B. Calcium ions, C. Strontium, D. Hydroxylions
D: Hydroxylions
Answer: D: Hydroxylions 
Explanation: None