20. Since the invention of digital readout, machine designers have rushed to replace conventional dials and gauges with digital units. Yet the digital gauge has drawbacks in some situations. Since it presents an exact numeric value, it must be decoded and analyzed by a human operator; its meaning cannot be read in an instantaneous scanning. An analog dial or gauge can be marked with red to alert the operator when a value is entering a danger zone; a digital gauge cannot. And it is difficult to tell whether a digital readout is increasing or decreasing over time, while the up or down movement of a pointer on an analog gauge can be quickly and easily observed.

The author of the passage above would probably recommend the use of digital gauge in cases when

I. warning of a sudden rise or fall in value is needed

II. an operator must read and interpret several gauges within a few seconds

III. a precise numeric value is essential

(A) I only

(B) III only

(C) I and III only

(D) II and III only（B）

(E) I, II, and III

答案为什么不是A？我认为III. a precise numeric value is essential正是digital gauge 已经具有的特征，从这句 Since it presents an exact numeric value, it must be .......即可看出。作者推荐使用digital gauge 只可能在弥补了这一缺点的情况下产生，即digital gauge 能象analog gauge 一样，能alert the operator when a value is entering a danger zone。

不知这种理解错在哪里？请各位指点。

digital gauge 虽能象analog gauge 一样，能alert the operator when a value is entering a danger zone，但需要解读，很花时间啊～～～

数字很抽象，不如图象直观。