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 of the following is used as, surface hardener in Impression material? 
Select and explain. 
A. 2% potassium sulfate, B. 0.2% potassium sulfate, C. 2% sodium sulfate, D. 4% potassium sulfate
A: 2% potassium sulfate
Answer: A: 2% potassium sulfate 
Explanation: None