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': 'Mode of action of systemic fluoride is: \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]