根据下面的文字资料回答 71～75 题
In the following essay，each blank has fourchoices．Choose the best answer and write down on thean，swer sheet．
Microwave communication uses（ 1 ）frequency radio waves that travel instraighdine sthroughthe air．Because the waves cannot（ 2 ）with the curvature of the earth，they can be （ 3 ）onlyover short distance．Thus，microwave is a good（ 4 ）for sending data between bulldingsin a city or on a large college campus．For longer distances，the waves must berelayed by means of“dishes”or（ 5 ）．These can be installed on towers，highbuildings，and mountain tops．
第71题：文中（ 1 ）处正确的答案是（ ）。
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