Definition: In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".
Input: [header] How to conduct a secondary survey of the neck, spine and back during first aid [title] Check your abc's. [substeps] Airway. The victim must have a patent airway. <sep> (A) They must not wear headwear or lean on their head during the procedure. Cover every part of the neck and spine. (B) If they are as porous as an ice pack but comfortable in the water and water, the ice will be able to penetrate the veins, mucous membranes, and liver. The same basic process applies if they have an airway. (C) There can be no obstruction and air should freely move in and out. Without a patent airway, nothing else matters. (D) An uncorrected airway is found in case your ambulance is on scene. Some states might allow one other person to operate the airway [title] Check the victim's head.
Output:
C