In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

Microwave communication uses (1) -frequency radio waves that travel in straightline sthrough the air. Because the waves cannot (2) with the curvature of the earth, they can be (3) only over short distance. Thus, microwave is a good (4) for sending data between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of" dishes" or (5) . These can be installed on towers, highbuildings, and mountain tops.

5（）

A．repeaters

B．radars

C．telephones

D．antennas

以下哪种人员是不需要进行任命的（）

A.安全*区代表

B.任务观察员

C.内审员

D.事故/事件调查员