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
The upper limit of urinary mercury level attributed to extensive amalgam filling is: 
Select and explain. 
A. 1 microgm/L, B. 2 microgm/L, C. 3 microgm/L, D. 4 microgm/L
D: 4 microgm/L
Answer: D: 4 microgm/L 
Explanation: None