Although the polar regions radiate away more heat energy than they receive by insolation in the course of a year, they are prevented from becoming progressively colder each year by the:

• A. Conduction of heat through the interior of the earth
• B. Concentration of earth's magnetic field lines at the poles
• C. Circulation of heat by the atmosphere and oceans
• D. The insulating properties of snow
• E. Release of latent heat to the atmosphere when polar ice melts