请作答74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

frequency

B

energy

C

amplitude

D

phase

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答案：

B

在题目的上下文中提到了"有限带宽"，这暗示了能量或功率的限制。在第74题中，提到方波有无穷多的频率成分，因此有无穷的能量。由于传输系统只能容纳有限的带宽，这意味着能量的限制。因此，正确答案是B，即能量。