This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mechanical injuries, Subject name: Forensic Medicine
[{'content': 'Not a pa of firearm - \nSelect and explain. \nA. Bolt, B. Piston, C. Extractor, D. Muzzle', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The synopsis of forensic medicine & toxicology ; Dr k.s narayan reddy ; 28 edition ; Pg no. 122,124,125 Refer the diagrams .', 'role': 'assistant'}]