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
[{'content': 'The basic constituent of reversible hydrocolloid impression material is: \nSelect and explain. \nA. Agar, B. Alginic acid, C. Gelatin, D. Alginate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Composition\nAgar,\xa0a\xa0polysaccharide\xa0extracted\xa0from\xa0certain\xa0types\xa0of 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\xa0strengthen 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.', 'role': 'assistant'}]