阅读下面的《三国演义》选段，根据原著的故事情节，回答问题。（5分）

操回顾玄德曰：“何如？”玄德答曰：“公不见丁建阳、董卓之事乎？”布目视玄德曰：“是儿最无信者！”（《三国演义》第十九回）

“丁建阳、董卓之事”具体指的是什么？为什么曹操要问刘备这句话？吕布为什么说刘备是“最无信者”？

答：                                                                                        

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.

（68）处填（）。

A．sequent

B．contiguous

C．following

D．successive