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
The discoloured corroded superficial layer frequently seen to the surface of amalgam restoration is most likely: 
Select and explain. 
A. Mercury, B. A sulfide, C. Gamma 2, D. Copper oxide
B: A sulfide
Answer: B: A sulfide 
Explanation: None