单选题 Digital data can also be represented by ______ signals by use of a modem. The modem converts a series of binary voltage ______ into an analog signal by encoding the digital data onto a carrier frequency. The resuming signal occupies a certain spectrum of ______ 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 ______ and hence allow those data to be propagated over ordinary voice-grade ______ lines. At the other end of the line, another modem demodulates the signal to recover the original data.