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': 'The chemical used to etch enamel is \nSelect and explain. \nA. Zinc oxide, B. methyl methacrylate, C. phosphoric acid, D. eugenol', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]