If you stand a short distance away from a high walland shout,your voice sometimes comes back to you as an echo.This knowledgewas made a few years before the last war,when radio waves from a transmittingstation were reflected back from a distant aeroplane.Itwas realized that if an apparatus(装置) could bedevised(设计) to send out radio waves all the time andrecord the echoes from any approaching aeroplanes,it would be a wonderfulmethod of defence in war.Scientistsset about solving the problem and radar was invented.An apparatus was made whichsent out waves in pulses ,and the presence of a distant aeroplane was on thescreen of a cathode ray(高速电子) tube,(this is what we havein a television set.) In this way radar station could detect the approach ofenemy aircraft.Afterthe war radar was put to peaceful uses.It is now installed at airports toguide aeroplane down safely in fog.It is fitted to ships so that they arewarned of obstructions(障碍物) ahead,such as icebergs Itguides ships into port.It provided aeroplane and ships with a magic eye,withwhich they can see far into the distance,in the dark or in fog.41.The last war (Line2 ,paragraph 1) may most probably refers to______.A.the World WarⅠ B.The Oct.RevolutionC.the World War Ⅱ D.the Chinese Liberation War42.Theinvention of radar______.A.has nothing to do with the reflection ofsound waves.B is exactly the same as the reflection ofsound wavesC.is on the basis of the reflection ofsound wavesD.is by changing sound waves into radiowaves43.The invention of radar is mainly for the purpose of______.A.scientific experiments B.defence in warC.peaceful uses D.finding enemy airplanes44.From the passage we may infer that radio waves can____.A.be reflected from almost everythingB.only be reflected from airplanesC.only be reflected from anything made of metalD.be reflected from anything not far away45.The word “detect” in the last sentence,the third paragraph most probablymeans______.A.hear B.smell C.feel D.discover

If you stand a short distance away from a high wall
and shout,your voice sometimes comes back to you as an echo.This knowledge
was made a few years before the last war,when radio waves from a transmitting
station were reflected back from a distant aeroplane.
It
was realized that if an apparatus(装置) could be
devised(设计) to send out radio waves all the time and
record the echoes from any approaching aeroplanes,it would be a wonderful
method of defence in war.
Scientists
set about solving the problem and radar was invented.An apparatus was made which
sent out waves in pulses ,and the presence of a distant aeroplane was on the
screen of a cathode ray(高速电子) tube,(this is what we have
in a television set.) In this way radar station could detect the approach of
enemy aircraft.
After
the war radar was put to peaceful uses.It is now installed at airports to
guide aeroplane down safely in fog.It is fitted to ships so that they are
warned of obstructions(障碍物) ahead,such as icebergs It
guides ships into port.It provided aeroplane and ships with a magic eye,with
which they can see far into the distance,in the dark or in fog.
41.The last war (Line2 ,paragraph 1) may most probably refers to______.
A.the World WarⅠ B.The Oct.Revolution
C.the World War Ⅱ D.the Chinese Liberation War
42.Theinvention of radar______.
A.has nothing to do with the reflection ofsound waves.
B is exactly the same as the reflection ofsound waves
C.is on the basis of the reflection ofsound waves
D.is by changing sound waves into radiowaves
43.The invention of radar is mainly for the purpose of______.
A.scientific experiments B.defence in war
C.peaceful uses D.finding enemy airplanes
44.From the passage we may infer that radio waves can____.
A.be reflected from almost everything
B.only be reflected from airplanes
C.only be reflected from anything made of metal
D.be reflected from anything not far away
45.
The word “detect” in the last sentence,the third paragraph most probably
means______.
A.
hear B.smell C.feel D.discover