Which Principle of Heat Exchange Best Explains Why Birds Fluff

Which principle of heat exchange best explains why birds fluff their feathers in colder weather?

A) Fluffing feathers results in less cooling by radiation because feathers emit less infrared radiation than other tissues do.
B) Fluffing decreases the amount of heat lost by conduction when the bird makes contact with cold objects in its environment.
C) Fluffing creates a pocket of air near the bird that acts as insulation.
D) Fluffing decreases the surface-area-to-volume ratio, thus decreasing the amount of heat lost to the environment.