Scientists 11 solving the problem and radar was 12. 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(阴极) ray tube, (this is 13 we have in a television set). 14 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 aeroplanes down safely in fog. It is fitted to ships so that they are warned 16 obstructions ahead,17 icebergs. It guides ships into port. It provides aeroplanes and ships 18 a magic eye, with 19 they can see 20 into the distance, in the dark or in fog.

　　1. A) voice B) sound C) shout D) echo

　　2. A) off B) on C) in D) against

　　3. A) so B) on C) but D) as

　　4. A) found B) discover C) knowledge D) discovery

　　5. A) when B) as C) until D) before

　　6. A) was B) were C) is D) are

　　7. A) if that B) if when C) that if D) when if

　　8. A) all time B) every time C) any time D) all the time

　　9. A) echoes B) voices C) sounds D) waves

　　10. A) on B) in C) at D) off

　　11. A) set off B) set aside C) set up D) set about

　　12. A) found B) seen C) made D) invented

　　13. A) what B) that C) which D) that

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