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': 'Acid etching of enamel margins \nSelect and explain. \nA. Provides chemical retention, B. Provides mechanical retention, C. Is used as resistance form, D. Is used for pulp protection', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]