题目内容
（请给出正确答案）
[主观题]
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(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
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