This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
[{'content': 'Stain for detecting early cancer in the oral and pharyngeal cavity is \nSelect and explain. \nA. Silver nitrate, B. Toluidine blue, C. Congo red, D. Zinc chloride', 'role': 'user'}, {'content': 'Answer: B \nExplanation: .', 'role': 'assistant'}]