Which principle of heat exchange is the most important explanation for why birds look larger in colder weather because they fluff their feathers?

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