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 basic constituent of reversible hydrocolloid impression material is: 
Select and explain. 
A. Agar, B. Alginic acid, C. Gelatin, D. Alginate
A: Agar
Answer: A: Agar 
Explanation: Composition
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.