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

LACE>I.LACE> 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

为什么不能选D呢,请NN指教