Digital data can also be represented by (71) signals by use of a modem.The modem converts a seriesof binary voltage (72) into

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问题   Digital data can also be represented by  (71)  signals by use of a modem.The modem converts a seriesof binary voltage  (72) into an analog signal by encoding the digital data onto a carrier frequency.The resuming signal occupies a certain spectrum of  (73) centered about the carrier and may be propagated across a medium suitable for that carrier.The most common modems represent digital data in the voice  (74)  and hence allow those data tO be propagated over ordinary voice-grade  (75)  lines.At the other end of the line,another modem demodulates the signal to recover the original data.
     (73)

选项 A、medium
B、frequency
C、modem
D、carrier

答案B

解析
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