问题
单项选择题
In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
Microwave communication uses high-frequency (26) waves that travel in straightlines through the air. Because the waves cannot (27) with the curvature of the earth, they can be (28) only over short distance. Thus, microwave is a good (29) for sendingdata between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of "dishes" or (30) . These can be installed on towers, high buildings, and mountain tops.
A．repeaters
B．radars
C．telephones
D．antennas
答案
参考答案：D
解析： 微波通信使用在空气中直线传播的高频电磁波，由于波不能随着地球的曲率弯曲，只能进行短距离的传播。因此，微波是一种在城市的建筑物或大学校园内发送数据的好介质。对于长距离，微波只能使用“dish”或者天线转播。这些设备被安装在塔顶、高的建筑物和山顶上。