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: Forensic Medicine
[{'content': 'An addicted patient presenting with visual and tactile hallucinations, has black staining of tongue and teeth. The agent is : \nSelect and explain. \nA. Cocaine, B. Cannabis, C. Heroin, D. Opium', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Cocaine', 'role': 'assistant'}]