True, my heat pump runs on electricity-an expensive commodity where I live. But most of its energy comes from the earth: a propylene-glycol solution circulates through 1,200 feet of two-inch-diameter plastic pipe buried three feet deep in a field next to my house. As it travels, the solution absorbs heat from the surrounding soil, even when soil temperature drops below freezing.
My ground-source heat pump also offers important benefits that have nothing to do with economics.
-Minimal fire hazard-no chimney, firebox, or heating elements-only pumps, fans, and a compressor.
-Cleanliness-no combustion products, thus no chance of my heating system polluting the indoor air, leaving scummy deposits on walls and furniture.
-Reliability-solid-state electronic controls and sealed beatings almost eliminate breakdowns.
-No maintenance-no wood to cut, ashes to haul, or chimney to clean.
-Convenience-The system runs automatically, even switching from heating to air conditioning as needed. I can simply lock the door and go away for a day, week, or month.
Not only did I expect to start saving on energy costs immediately, but I also expected those savings to grow over the years as oil process continued to soar. Since I was wrong about oil prices, the big dollar savings I' d hoped for haven' t materialized. Nevertheless, I am satisfied, all things considered.
The passage suggests that the author' s heat-pump system absorbs heat from ______.

A. soil in a nearby field
B. sun panels at ground level
C. a pipe buried next to the walls
D. a tank of propylene glycol