Correct Response: D. Geothermal heating and cooling systems rely on the relatively stable temperature twenty to thirty feet underground. Although the temperature varies geographically with latitude, temperatures at this depth do not vary much seasonally and are never below freezing. This provides a source of heat in the winter and in the summer the same system can be used for cooling. Temperatures deep underground are hot (A), but the depth is too great to provide an economically feasible source of heat energy for small-scale geothermal air-conditioning systems. Compression of water vapor (B) is used in heat pumps to increase the temperature of the water coming out of the ground not going into the ground. Soils are too close to the surface to be a heat source in the winter or a heat sink in the summer (C).
