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 highest mercury concentration in amalgam filling is found \nSelect and explain. \nA. At the margin of the restoration, B. In the centre of the restoration, C. In the deepest part of the restoration, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]