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【单选题】
以30m/s 的速度运动的汽车,鸣笛时,喇叭的振动频率为500Hz 。汽车正后方声波的波长是 (空气中的声速为330m/s )A.
0.72m
B.
0.6m
C.
0.55m
D.
0.79m
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举一反三
【单选题】The author has the approach of a (n) _______.
A.
scientist
B.
artist
C.
novelist
D.
economist
【简答题】简述调度命令应包括的内容
【单选题】What does the author mean by 'back care has gone astray' in the last paragraph?
A.
It has been on the right track.
B.
It has been away from the right track.
C.
It has offered a new way of treatment.
D.
It has moved back to tradition.
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