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
Mode of action of systemic fluoride is: 
Select and explain. 
A. Formation of protective layer over tooth, B. Inhibit enzymatic breakdown of enamel, C. As F— content increases, solubility of enamel decreases, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None