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 tarnished layer of silver amalgam consists of: \nSelect and explain. \nA. Sulphides of silver, B. Oxides of tin, C. Chlorides of tin, D. All of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]