The following are multiple choice questions (with answers) about medical knowledge.   **Question:** Which ion of hydroxyapatite crystals is substituted by Fluoride imparts more strength?   (A) Hydroxyl ion   (B) Calcium ion   (C) Phosphate ion   (D) None
['(A) Hydroxyl ion <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Hydroxyl ion \n (B) Calcium ion \n (C) Phosphate ion \n (D) None <> possible answers', '  Which ion of hydroxyapatite crystals is substituted by Fluoride imparts more strength? <> context@']