You arrive on scene to find an unresponsive homeless man lying in the alleyway clutching a wine bottle. He is wearing a​ long-sleeved shirt but does not have pants on. His skin feels very cold to the​ touch, and he does not have discernible vital signs. The​ week's temperatures have not been above 50 degrees F. Which of the following should be done​ FIRST?

A. Ensure an open airway.
B. Begin passive rewarming.
C. Check a carotid pulse.
D. Apply the AED.