This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Surgery
X-ray sign of pneumoperitoneum is 
Select and explain. 
A. Seagull sign, B. Prehn sign, C. Rigler's sign, D. Golden 'S' sign
C: Rigler's sign
Answer: C: Rigler's sign 
Explanation: Rigler's sign - bowel wall outlined by intraluminal and free peritoneal gas.