Select the best definition of Postoperative atelectasis.

A. Inflammation of the gallbladder, commonly due to cystic-duct obstruction by a stone. B. Systematic evaluation of surgical risk, comorbidity, medicines, airway and planned procedure. C. Collapse of alveoli after surgery, promoted by pain, shallow breathing and immobility. D. Raised pressure within a closed fascial compartment that threatens tissue perfusion.