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': 'According to the Nathoo classification, the chromogen changes color after binding to the tooth is categorized under: \nSelect and explain. \nA. N1 type stain, B. N2 type stain, C. N3 type stain, D. N4 type stain', 'role': 'user'}, {'content': 'Answer: B \nExplanation: N1 type or direct dental stain:\xa0\nThe colored materials (chromogen) bind to the tooth surface and cause\xa0 discoloration. The color of the dental stain is same as the color of the chromogen.\nN2 type or direct dental stain:\xa0\nThe chromogen changes color after binding to the tooth.\nN3 type or indirect dental stain:\xa0\nColorless material or a prechromogen binds to the tooth and\xa0 undergoes chemical reaction to cause a stain.\nTextbook of Endodontics Nisha GARG 3rd ED', 'role': 'assistant'}]