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.