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': 'By increasing the percentage of which metal, the strength and hardness of amalgam increases: \nSelect and explain. \nA. Ag, B. Zn, C. Cu, D. Hg', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]