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 discoloured corroded superficial layer frequently seen to the surface of amalgam restoration is most likely: \nSelect and explain. \nA. Mercury, B. A sulfide, C. Gamma 2, D. Copper oxide', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]