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': 'A black line on the gingiva which follows the contour of the margin is due to: \nSelect and explain. \nA. Lead, B. Argyria, C. Iron, D. Mercury', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]