Answer-A. PneumoperitoneumMC cause of pneumoperitoneum: Perforation of hollow viscus (leading to release of air from bowel and collection just below the diaphragm)Best projection to derhonstrate pneumoperitoneum: Chest X-rayIf the patient cannot get into an erect position then left lateral decubitus projection is required.Patient should be in that position for l0 mina at least for air to rise up.By careful technique even 1 ml of air can be detected