本题是2008年下半年学位英语教材第十单元的课后完型填空。

本题是2009年下半年学位英语教材第十单元的课后完型填空。

If you stand a short distance away from a high wall and shout, your voice sometimes comes, back to you as an echo. This happens because the 1.sound waves are reflected 2.off the wall. This is the principle of radar, 3.but instead of sound it is radio waves which are reflected. This 4.discovery was made a few years before the last war, 5.when radio waves from a transmitting station 6.were reflected back from a distant aeroplane.

It was realized 7.that if an apparatus could be devised to send out radio waves 8.all the time and record the 9.echoes from any approaching aeroplanes, it would be a wonderful method of defense in 10.war.

Scientists 11.set about solving the problem and radar was 12.invented. An apparatus was made which sent out waves in pulses, and the presence of a distant aeroplane was shown on the screen of a cathode ray tube,(this is 13.what we have in a television set).14.In this way radar stations could detect the approach of enemy aircraft.

After the war radar was put to peaceful uses. It is now installed at airports to 15.guide aeroplanes down safely in fog. It’s fitted to ships so that they are warned 16.of obstructions ahead, 17.such as icebergs. It guides ships into port. It provides aeroplanes and ships 18.with a magic eye, with 19. which they can see 20.along into the distance, in the dark or in fog.

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