阅读理解 In 1983, when oil process were till high in my area, I installed a ground-source heat pump in my brand-new, and insulated 1,600-square-foot house. It cost several thousand dollars more than comparable systems that use oil, gas, or wood, but I figured I''d recover my higher initial expense in a few years through lower energy bills. 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.

单选题 The passage suggests that the author'' s heat-pump system absorbs heat from ______.

单选题 One benefit the author mentions is that the system______.

单选题 The best title for the passage is ______.