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【单选题】
What does the speaker say about this year's business travellers?A.
There are fewer business travellers.
B.
There are more business travellers.
C.
The number remains the same as last year's.
D.
It's not mentioned in the passage.
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举一反三
【单选题】某正弦电压的最大值为311V,周期为0.02s、初相为30°,则该正弦电压的表达式为
A.
Au=220sin（314t-30°）V
B.
Bu=220sin（314t+30°）V
C.
Cu=311sin（314t-30°）V
D.
Du=311sin（314t+30°）V
【简答题】已知：a，b，c都是正实数，且ab+bc+ca=1．求证： ．
【单选题】What is an advantage of an externally modulated laser over a directly modulated laser?（）
A.
lower cost
B.
higher chirp
C.
easier to manufacture
D.
higher dispersion tolerance
E.
higher optical signal-to-noise ratio
【判断题】The envelope of the AM signal can fully reflect the information of the modulated signal when the AM Ma is less than 1. ( )
A.
正确
B.
错误
【单选题】Which of the following signals carries modulated signal information ( ).
A.
Carrier signal
B.
L ocal oscillator signal
C.
Modulated signal
D.
above are all wrong
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