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': 'Air abrasion \nSelect and explain. \nA. 27 micron alumina, B. 100 micron alumina, C. 27 micron zirconia, D. 100 micron zirconia', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]