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 upper limit of urinary mercury level attributed to extensive amalgam filling is: \nSelect and explain. \nA. 1 microgm/L, B. 2 microgm/L, C. 3 microgm/L, D. 4 microgm/L', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]