He had to pause from time to time to wipe the sweat from his forehead, because the air-conditioning system ______________.A.broke inB.broke upC.broke outD.broke down
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D解析考查动词短语辨析。break in打断； 闯入； 开始工作；break up结束，散开；分解；break out爆发；break down崩溃，垮掉；出毛病；抛锚。题干意思是：他时不时地停下来擦去额头的汗水，因为空掉出毛病了。选D。