Microwave communication uses high-frequency(71)waves that travel in straightlines through the air. Because the waves cannot(72)with the curvature of the earth, they can be(73)only over short distance. Thus, microwave is a good(74)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(75). These can be installed on towers, highbuildings, and mountain tops.
A.optical
B.radio
C.electrical
D.magnetic
第4题
A.闸机所有扇门将保持开放状态;
B.所有的乘客显示器显示紧急放行信息;
C.所有在向付费区的末端指示器闪烁显示“通行”标志;
D.所有在向非付费区的末端指示器闪烁显示“禁止通行”标志
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