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': 'Chlorhexidine on tooth causes \nSelect and explain. \nA. No stain, B. Brownish yellow stains, C. Greenish yellow stain, D. Blue back stain', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]