2. 计算机
  3. In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet. Microwave com

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

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问题 In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
   Microwave communication uses(66)-frequency radio waves that travel in straightline sthrough the air. Because the waves cannot(67)with the curvature of the earth, they can be(68)only over short distance. Thus, microwave is a good(69)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(70). These can be installed on towers, highbuildings, and mountain tops.
选项 A、repeaters
B、radars
C、telephones
D、antennas
答案D
解析 微波通信利用在空气中直线传播的高频无线电波通信，因为电磁波不能随着地球表面的曲度而弯曲，所以只能用来在短距离内传输。这样，微波就是一种在大楼之间或在大的大学校园里用来发送数据的优良介质。如果要进行更长距离的传输，电磁波必须通过蝶行卫星天线或普通天线中转。这些天线能够被安装在塔顶、高楼和山顶上。
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