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: Radiology
[{'content': "Rigler's triad is seen in \nSelect and explain. \nA. Pneumo peritoneum, B. Intestinal obstruction, C. Gallstone ileus, D. Ulcerative colitis.", 'role': 'user'}, {'content': "Answer: C \nExplanation: Gallstone ileus - Rigler's triad\n\nEctopic gall stone\nPneumobilia\nSmall bowel obstruction.", 'role': 'assistant'}]