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
2% solution of potassium sulfate is used as a surface hardening agent for: 
Select and explain. 
A. Casting stone, B. Dental plaster, C. Dental stone, D. Type IV gypsum
B: Dental plaster
Answer: B: Dental plaster 
Explanation: Agar, a polysaccharide extracted from certain types of seaweed, is the main active ingredient in the material, while water is the major constituent in the agar impression material. A small percentage of borax is added to strengthen the gel. Since borax is a potent retarder for the setting of gypsum, an accelerator such as potassium sulfate is added to counteract the effect of borax.