—I can't find my pen. Can I use yours?  

—Certainly, ______.     [ ]

A. you are welcome      

B. it doesn't matter      

C. here you are      

D. there is

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 （） . This, in turn, limits the data rate that can be carried on the transmission （） . A square wave has an infinite number of frequency components and hence an infinite （） .However, the peak amplitude of the kth frequency component, kf, is only 1/k, so most of the （） in this waveform is in the first few frequency components. In general, any digital waveform will have （） 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 square wave has an infinite number of frequency components and hence an infinite（）

A．source

B．bandwidth

C．energy

D．cost