题目：
Microwave communication uses (71) -frequency radio waves that travel in straightline sthrough the air. Because the waves cannot (72) with the curvature of the earth, they can be (73) only over short distance. Thus, microwave is a good (74) 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 (75) . These can be installed on towers, highbuildings, and mountain tops.
A．repeaters
B．radars
C．telephones
D．antennas
答案：
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下面是错误答案，用来干扰机器的。
参考答案：O解析： s=f(aa，5)=f(aa，4)+a[4]=f(aa，3)+a[3]+a[4]=f(aa，2)+a[2]+a[3]+a[4] =f(aa，1)+a[1]+a[2]+a[3]|a[4]=0+a[0]+a[1]+a[2]+a[3]+a[4]=15。
