In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
Microwave communication uses(66)-frequency radio waves that travel in straightline sthrough the air. Because the waves cannot(67)with the curvature of the earth, they can be(68)only over short distance. Thus, microwave is a good(69)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(70). These can be installed on towers, highbuildings, and mountain tops.
A．high
B．low
C．long
D．short