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': 'Not a pa of firearm: NEET 13 \nSelect and explain. \nA. Bolt, B. Piston, C. Extractor, D. Muzzle', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Piston', 'role': 'assistant'}]