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': 'Discolouration of silver alloy is due to: \nSelect and explain. \nA. Tarnish, B. Tarnish and corrosion, C. Wet corrosion, D. Dry corrosion', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]