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 ion of hydroxyapatite crystals is substituted by Fluoride imparts more strength? 
Select and explain. 
A. Hydroxyl ion, B. Calcium ion, C. Phosphate ion, D. None
A: Hydroxyl ion
Answer: A: Hydroxyl ion 
Explanation: None