For more than twenty years scientists have been searching for signs of life on other planets. Most of these searches have been done over the radio, a scanning of the electromagnetic waves(電磁波)that come to us from outer space. Scientists also have sent radio and television messages, as well as messages on spacecraft traveling through space, hoping that someone may receive such messages.

       The most extensive search so far involves using powerful radio telescopes to listen to signals from about 1,000 stars, all within 100 light years of Earth. In addition, astronomers will scan the entire sky to “l(fā)isten” for radio messages from more distant stars. Scientists are looking for any signal that stands out from the background noise, such as a strong tone that could have traveled great distances.

Of the 200 billion stars in the Milky Way galaxy（星系）, scientists find that five percent are like our sun. Of these solar systems, perhaps half of them have a planet like Earth. Such a planet would be a reasonable distance from the star for temperatures to be right for the evolution of life. Based on the inhabitable planets in our galaxy, most scientists agree that chances are likely that one or more of these planets support some life.

       However, many scientists wonder whether intelligent life exists on other planets. Some believe that twenty years of searching shows that no one is out there. They say that the evolution of intelligence similar to ours is unlikely. Insects, perhaps the most evolutionarily successful of all living things on Earth, for instance, could not communicate over radio waves.

       Other scientists believe that our search has been neither long enough nor extensive enough to rule out(拒絕考慮) the possibility that intelligent life exists in our galaxy. Although our solar system is only about five billion years old, our galaxy is about 20 billion years old. In that time some scientists think it is likely that civilizations much more advanced than ours have developed. Perhaps these civilizations are sending us no signals; perhaps we have not recognized the signals they have sent us. If we hope to find intelligent life, these scientists believe that we have to keep looking.

1．According to the passage, ____stars might have a planet like Earth.    

       A．a(chǎn)ll                       B．5 billion               C．10 billion             D．200 billion

2．The first paragraph in this passage is mainly about            .

       A．how scientists are looking for signs of life on other planets

       B．why scientists are looking for signs of life on other planets

       C．how to use electromagnetic waves to search outer space

       D．what scientists have done with radio and television messages

3．Which of these statements is TRUE based on the information in the passage?   

       A．The earth is one of the oldest planets in our galaxy.

       B．Most scientists believe that there is intelligent life on other planets.

       C．Scientists are trying different ways to find signs of life on other planets.

       D．Scientists think that the evolution of intelligence on other planets is similar to that on earth.

4．Why do some scientists think there might be more advanced civilization on other planets?                       

       A．Because some planets in our galaxy are much older than Earth.

       B．Because civilization on earth is likely to have more time to develop.

       C．Because scientists prove it with the aid of the scanning of the electromagnetic waves.

       D．Because there are many more stars on other planets.

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