<s>[INST] context: The term pneumoperitoneum refers to the presence of air within the peritoneal cavity. The most common cause is a perforation of the abdominal viscus--most commonly, a perforated ulcer. At least 2 radiographs should be obtained, including a supine abdominal radiograph and either an erect chest image or a left lateral decubitus image. The patient should remain in position for 5-10 minutes before a horizontal-beam radiograph is acquired. A lateral chest x-ray has been found to be even more sensitive for the diagnosis of pneumoperitoneum than an erect chest x-ray. Signs of a large pneumoperitoneum include the following: The football sign, which usually represents a large collection of air within the greater sac. The gas-relief sign, the Rigler sign, and the double-wall sign are all terms applied to the visualization of the outer wall of bowel loops caused by gas outside the bowel loop and normal intraluminal gas. The lateral umbilical ligaments, which contain the inferior epigastric vessels, may become visible as an inveed V sign in the pelvis as a result of a large pneumoperitoneum. A telltale triangle sign represents a triangular pocket of air between 2 loops of bowel and the abdominal wall. Free air under the diaphragm may depict the diaphragmatic muscle slips as arcuate soft tissue bands, arching parallel to the diaphragmatic dome. Air may be present around the spleen. [/INST] question: Which is the best investigation to detect Pneumoperitoneum? | options: A: Plain X ray abdomen, erect B: Left lateral decubitus C: Plain X ray chest, erect D: CT Scan | answer: C </s>