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': 'Recommended time for acid etching before placing a pit and fissure sealant \nSelect and explain. \nA. 30 sec., B. 60 sec., C. 90 sec., D. 120 sec.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]