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：C
解析：
参考译文
一般来说，“字符”是用于对数据进行表示、组织或控制的元素集合中的一个成员。字符可分为“图形字符”与“控制字符”两大类。
与控制字符不同的是：图形字符由其视觉表示，正常情况下通过写出、打印或显示产生。图形字符包括数码字符、字母字符、表意字符和专用字符。
控制字符出现在某一特定上下文中，以此规定某一控制功能。控制字符包括传输控制字符、格式控制字符、代码扩展字符和设备控制字符。可将一个控制字符记录下来，供后续动作使用。控制字符虽然不是图形字符，但在某些情况下可以由某种图形表示。