请作答72题：Although a given waveform may contain frequencies over a very broad range, as a practical matter any transmission system will be able to accommodate only a limited band of （71） . This, in turn, limits the data rate that can be carried on the transmission （72） . A square wave has an infinite number of frequency components and hence an infinite （73） .However, the peak amplitude of the kth frequency component, kf, is only 1/k, so most of the （74） in this waveform is in the first few frequency components. In general, any digital waveform will have （75） bandwidth. If we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

A

procedures

B

function

C

route

D

medium

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答案：

D

题目中的关键词是"transmission system"，意为传输系统，后面提到的是一个有限的传输带宽。选项D中的medium意为媒介或介质，与传输系统在传输信息时使用的媒介相符合。因此，正确答案是D。