单项选择题 What do plants do when they are thirsty?（）

单项选择题 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.A．acquireB．collectC．assumeD．fetch

单项选择题 公称容积小于1000m3的球形罐常用的组装方法是（）。