In the following essay, each blank has four choices. Choose the best answer and writedown on the answer sheet.
Microwave communication uses high-frequency (26) waves that travel in straightlines through the air. Because the waves cannot (27) with the curvature of the earth, they can be (28) only over short distance. Thus, microwave is a good (29) for sendingdata between buildings in a city or on a large college campus. For longer distances, the wavesmust be relayed by means of "dishes" or (30) . These can be installed on towers, highbuildings, and mountain tops.
（29）是（）
A.material
B.equipment
C.medium
D.channel
面中心受力时，两端基牙上的