The scientists did not tell people about the signals because____.

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• 1The radio singals were sent by____.

  A.a satellite

  B.a planet

  C.a sky body which was unknown at that time

  D.intelligent beings who were unknown at that time 

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• 2The radio signals discussed in this passage____.

  A.were regular

  B.were controlled by a clock

  C.were heard in 1967 only

  D.were secret messages 

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• 3A pulsar is____.

  A.a small heavy star which sends out strong radio signals and cannot be seen 

  B.a small heavy planet which sends out strong radio signals and cannot be seen 

  C.a small heavy satellite which sends out strong radio signals and cannot be seen

  D.a small intelligent being who sends out strong radio signals and cannot be seen

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• 4Which of the following is true、

  A.One of the pulsars found by scientists sends radio signals.

  B.Pulsar began to send radio singals in 1967.

  C.Scientists have searched for pulsars for many years but found none.

  D.Scientists have found many pulsars since 1967. 

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• 5What is the article mainly concerned、

  A.The problem of spreading desert.

  B.The rainfall and vegetation in desert areas.

  C.The water collection systems.

  D.The difference between modern life and ancient lif

  E.

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• 6“one out of seven” refers to____.

  A.more than a third of the lands' earth

  B.the percentage of the earth's land that is desert-like

  C.the number of people who live in dry regions

  D.a day of a week 

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• 7In paragraph 2,“they are taken to the greener lands in the south.”Theyrefers to____.

  A.the Sahel farm land

  B.the farmers

  C.the cattles

  D.the trees 

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