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 is done for \nSelect and explain. \nA. 30 Seconds, B. 60 seconds, C. 90 seconds, D. 120 seconds', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]