【题目】He had to pause from time to time to wipe the sweat from his forehead, because the air-conditioning system ______________.
A.broke in
B.broke up
C.broke out
D.broke down

【答案】D
【解析】句意：他时不时地停下来擦去额头的汗水，因为空调出毛病了。A.break in“打断， 闯入，开始工作“；B.break up”结束，散开，分解“；C.break out”爆发“；D.break down”崩溃，垮掉“。根据句意，选D。