下列有关声现象的说法中，正确的是                                  （    ）

A．声波能传递信息，也能传递能量

B．“禁止高声喧哗”中的“高”是指声音的音调高

C．公路旁的隔音墙是在声源处减弱噪声

D．两名宇航员在太空中能直接对话

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.

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 （） .

A．frequencies

B．connections

C．diagrams

D．resources