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 thin bluish line around the gingival margin is due to\nthe absorption of \nSelect and explain. \nA. Silver, B. Mercury, C. Drugs like tetracyclines, D. Lead', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]