2016公共英语二级(PETS2)考试模拟试卷九(7)免费下载
度) that both heats and cools. In summer, a heat pump is a conventional (普通的，习惯的) air conditioner which removes heat from a building. In winter, the system reverses itself to use the natural heat that is always present in outdoor air. By taking advantage of the free heat, a heat pump is more econom
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