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
The impression material that is mainly composed of sodium or potassium salts of alginic acid is _____. 
Select and explain. 
A. Polyether, B. Irreversible hydrocolloid, C. Polyvinyl siloxane, D. Polysulfide
B: Irreversible hydrocolloid
Answer: B: Irreversible hydrocolloid 
Explanation: Irreversible hydrocolloid (IH) or alginate is the material of choice to produce diagnostic casts. Its composition is mainly sodium or potassium salts of alginic acid. They react chemically with calcium sulfate to produce insoluble calcium alginate.