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
The primary action of topical fluoride is a: 
Select and explain. 
A. Conversion of hydroxyapatite to fluoroapatite, B. Decrease in the plaque bacteria, C. Form a reservoir in saliva, D. Improve morphology of teeth
A: Conversion of hydroxyapatite to fluoroapatite
Answer: A: Conversion of hydroxyapatite to fluoroapatite 
Explanation: None