云能悬浮在空气中的原因是什么？
参考答案：
云能悬浮在空气中是由于云粒的下降速度小于空气的上升速度。
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
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