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.
（70）处填（）。
A．limited
B．finite
C．restricted
D．bounded
参考答案：B
解析：
[参考译文]
模拟信号表示数据的特征量，在任一时刻，可取某一连续区间内的任一值。例如，模拟信号可以连续地跟随表示数据的另一物理量的值。
离散信号由时间上相继的各元素组成，其中每一元素都有一个或多个能表示数据的特征量。特征量的例子有振幅、波形、持续时间和当时位置。数字信号是离散信号的一种特例，其中的数据由已明确定义、个数有限的离散值来表示，各值均可由该信号的特征量之一随时间来取。