It is late Friday evening and you are joining your friends at a cabin for a winter weekend trip. When you open the door you notice how nice and warm it is inside. Your friends though are not very welcoming. They tell you they are not feeling well and complain of being lightheaded and nauseated. Based on their symptoms, you are concerned they may be suffering from:

a. carbon dioxide poisoning.
b. alcohol toxicity.
c. excessive heat exposure.
d. carbon monoxide poisoning.

Answer: d