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
An addicted patient presenting with visual and tactile hallucinations, has black staining of tongue and teeth. The agent is - 
Select and explain. 
A. Cocaine, B. Cannabis, C. Heroin, D. Opium
A: Cocaine
Answer: A: Cocaine 
Explanation: None