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
Which is used for etching porcelain? 
Select and explain. 
A. 9.6% HF, B. 1.23% HF, C. 37% HF, D. 4% HF
A: 9.6% HF
Answer: A: 9.6% HF 
Explanation: None