单选题 24.  John had to pause from time to time to wipe the sweat off his forehead, because the air-conditioning system ______.
【正确答案】 D
【答案解析】[考点] 词义辨析
break in：打断；break out：爆发；break up：打碎，分手；break down：(机器等)出现故障。根据句意可知，选D。
   因为空调系统出了故障，约翰不得不一次次停下来擦掉额头上的汗水。