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
Air abrasion 
Select and explain. 
A. 27 micron alumina, B. 100 micron alumina, C. 27 micron zirconia, D. 100 micron zirconia
A: 27 micron alumina
Answer: A: 27 micron alumina 
Explanation: None