This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Peritoneum, Subject name: Surgery
[{'content': 'Best investigation for air in peritoneal cavity is: \nSelect and explain. \nA. USG, B. Laparotomy, C. Laparoscopy, D. X-ray', 'role': 'user'}, {'content': "Answer: D \nExplanation: A radiograph of the abdomen may confirm the presence of dilated gas-filled loops of bowel (consistent with a paralytic ileus) or show free gas, although the latter is best shown on an erect chest radiograph. If the patient is too ill for an 'erect' film to demonstrate free air under the diaphragm, a lateral decubitus film is just as useful, showing gas beneath the abdominal wall.", 'role': 'assistant'}]