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
[{'content': 'Black line in the gingiva that follows the contour of the margin is due to: \nSelect and explain. \nA. Bismuth, B. Arsenic, C. Mercury, D. All of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Black line in the gingiva\xa0that follow the contour of the margin is due to Bismuth, Arsenic and Mercury.', 'role': 'assistant'}]