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
[{'content': "X-ray sign of pneumoperitoneum is \nSelect and explain. \nA. Seagull sign, B. Prehn sign, C. Rigler's sign, D. Golden 'S' sign", 'role': 'user'}, {'content': "Answer: C \nExplanation: Rigler's sign - bowel wall outlined by intraluminal and free peritoneal gas.", 'role': 'assistant'}]