Read lines 45 through 49. While temperatures above ground change a lot from day to day and season to season,
temperatures 10 feet below the Earth's surface hold nearly constant between 50° and
60°F. For most areas, this means that soil temperatures are usually warmer than the air in winter and cooler than the air in summer. Geothermal heat pumps use the Earth's constant temperatures to heat and cool buildings. They transfer heat from the ground (or water) into buildings in winter and reverse the process in the summer.
Why does the author discuss the transfer of heat in the last sentence of the paragraph?
A to explain how the heat pump can help people stay warm and cool
B to convince the reader that a heat pump is a worthwhile investment
C to show what makes geothermal activity happen under the Earth's crust
D to argue that geothermal energy is superior to gas or electricity