Microwave communication uses high-frequency (56) waves that travel in straightlines through the air. Because the waves cannot (57) with the curvature of the earth, they can be (58) only over short distance. Thus, microwave is a good (59) 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 (60) .These can be installed on towers, high buildings, and mountain tops.

A．optical

B．radio

C．electrical

D．magnetic

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概括这一节的中心意旨。