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
Not a pa of firearm - 
Select and explain. 
A. Bolt, B. Piston, C. Extractor, D. Muzzle
B: Piston
Answer: B: Piston 
Explanation: The synopsis of forensic medicine & toxicology ; Dr k.s narayan reddy ; 28 edition ; Pg no. 122,124,125 Refer the diagrams .