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: Pathology
Black line in the gingiva that follows the contour of the margin is due to: 
Select and explain. 
A. Bismuth, B. Arsenic, C. Mercury, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Black line in the gingiva that follow the contour of the margin is due to Bismuth, Arsenic and Mercury.