8. 修辞目的性题

Xerography

1    One more familiar use of electrochemistry that has made its way into the mainstream is xerography, a process for replicating documents that is dependent on photoconductive materials. A photoconductive material is an insulator in the dark but becomes a conductor when exposed to bright light. When a photocopy is being made, an image of a document is projected onto the surface of a rotating drum, and bright light causes the photoconductive material on the surface of the drum to become conductive.

2    As a result of the conductivity, the drum loses its charge in the lighted areas, and toner (small grains to which dry ink adheres) attaches itself only to the darker parts of the image. The grains are then carried to a sheet of paper and fused with heat. When a laser printer is used, the image is projected by means of a laser beam, which creates a brighter light and a greater contrast between lighter and darker areas and therefore results in sharper printed images.

1. The author begins the first paragraph with One more familiar use of electrochemistry in order to

  (A) explain that xerography is one of the less familiar uses of electrochemistry

  (B) make it clear that electrochemistry requires photoconductive materials

  (C) show that xerography is the only known use for electrochemistry

  (D) indicate that other less familiar uses have already been discussed

2. Why does the author explain that A photoconductive material is an insulator in the dark but becomes a conductor when exposed to bright light?

  (A) It gives an explanation of a property that is necessary for xerography.

  (B) It indicates that bright light is required for insulation to take place.

  (C) It gives one example of a successful xerographic process.

  (D) It explains the role of insulation in xerography.

3. The author places the phrase small grains to which dry ink adheres in parentheses in order to

  (A) provide information that contradicts the previous statement

  (B) provide another example of conductivity

  (C) provide further detail information about toner

  (D) provide an alternate explanation for the effectiveness of toner

4. Why is a laser printer mentioned?

  (A) It is an alternative to xerography.

  (B) It is a way of duplicating without using electrochemistry.

  (C) It is a second example of xerography.

  (D) It is a less effective type of xerography than is a photocopier.

正确答案：D_A_C_C