On the playground,we saw many boys    basketball.

A．playing

B．played

C．played the

D．stayed

An analog signal’s characteristic quantity representing data may (66) . at any instant, any value within a continuous (67) . For example, an analog signal may follow continuously the values of another physical quantity representing data.

A discrete signal is composed of (68) elements in time, each element having one or more characteristic quantities which can represent data. Examples of characteristic quantities are amplitude, waveform, (69) , and position in time. A digital signal is a special case of discrete signals, in which data are represented by a (70) number of well-defined discrete values that one of its characteristic quantities may take in time.

（69）处填（）。

A．duration

B．period

C．persistence

D．span