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
Etching depth after 30 sec for orthodontic bracket with orthophosphoric acid is: 
Select and explain. 
A. 10 μm — 20 μm, B. 3 μm — 9 μm, C. 50 μm — 110 μm, D. 200 μm — 250 μm
A: 10 μm — 20 μm
Answer: A: 10 μm — 20 μm 
Explanation: None