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:

The correct answer is: C. Circulation of heat by the atmosphere and oceans.

The atmosphere and oceans are constantly moving, and this movement helps to distribute heat around the globe. The warm air from the equator rises and moves towards the poles, where it cools and sinks. This process, known as convection, helps to keep the polar regions from becoming too cold.

The other options are incorrect because:

• A. Conduction of heat through the interior of the earth is a very slow process, and it would not be enough to prevent the polar regions from becoming progressively colder each year.
• B. Concentration of earth’s magnetic field lines at the poles does not have a significant effect on the temperature of the polar regions.
• D. The insulating properties of snow help to keep the ground warm, but they do not prevent the polar regions from radiating away heat energy.
• E. Release of latent heat to the atmosphere when polar ice melts does help to warm the polar regions, but it is not enough to prevent them from becoming progressively colder each year.