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': 'Enamel crystallites are stabilized against acid if fluoride ions replace \nSelect and explain. \nA. Phosphate ions, B. Calcium ions, C. Strontium, D. Hydroxylions', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]