答案：A

　　19．A.for

　　B．therefore

　　C．but

　　D．as

　　解析：句中的两个宾语从句是转折关系，所以用but连接。

　　答案：C

　　20．A.guide

　　B．control

　　C．mix

　　D．ready

　　解析：因为是做试验，有一组猴子在食量方面是受到“控制的”。

　　答案：B

　　Ⅲ.阅读理解

　　A

　　The sky is a huge chessboard for air traffic controllers. In 314 control towers and navigation(导航) centers across the country, the air traffic controllers comfortably sit down in windowless rooms, staring at radar screens that show aircraft moving through

　　thousands of square miles. Radio headsets make nonstop noises with pilots requesting course changes, asking for greater or smaller heights, and, on a bad day, declaring emergencies. Each controller communicates with as many as 30 aircraft at a time, giving commands that direct each plane's movement.

　　A half century ago, flying was a much simpler affair. Pilots navigated by pointing their planes where they wanted to go, and it was up to the captain to avoid running into other aircraft in the sky. But with the increase in air travel following World War Ⅱ， the government stepped into regulate­control with rules­flight paths around major airports, and the responsibility for navigation changed from pilots to air traffic controllers.

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