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【单选题】
扫描时间因数为 10ns/div,被测脉冲在水平轴上显示一个完整周期为5div,则该的频率是( )A.
2 0MHZ
B.
50MHZ
C.
20KHZ
D.
50KHZ
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【简答题】为什么虞世南被称为三朝五代元老?
【单选题】Sorry, Peter. I took your backpack . [ ]
A.
by accident
B.
by the way
C.
by air
D.
by mistake
【单选题】Air and water __ are still serious problems in many developing countries. [ ]
A.
pollution
B.
accident
C.
development
D.
height
【单选题】There was an air crash in Xi'an, the pilot didn't __________ the accident. [ ]
A.
survive from
B.
survive in
C.
survive of
D.
survive
【单选题】According to the passage, if an air plane has an accident-free record, it usually pays to the insurance company ______.
A.
a large premium
B.
less premium
C.
no premium
D.
the same premium as the other companies
【单选题】）Tong is afraid ,because he was terrified by an air accident．
A.
fly
B.
of flying
C.
to flying
D.
flying
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