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  3. For more than thirty years scientists have been searching for signs of life on other planets. Most of these searches have been d

For more than thirty years scientists have been searching for signs of life on other planets. Most of these searches have been d

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问题    For more than thirty years scientists have been searching for signs of life on other planets. Most of these searches have been done over the radio. The hope is that someone in outer space may be trying to get in touch with us. Scientists also have sent radio and television messages, as well as messages on spaceships traveling through space, on the chance that someone may be receptive to such messages.
   Scientists are using powerful radio telescopes to "listen" to signals from about 1,000 stars, all within 100 light years of the earth. In addition, they will scan the entire sky to listen for radio messages from more distant stars. Using a computer, they will be able to monitor more than eight million channels at a time. Scientists are looking for any signal that stands out from the background noise.
   Of the 200 billion stars in the Milky Way galaxy, scientists find that five percent of the stars are like our sun. Perhaps half of them have a planet like the earth. Such a planet would be a reasonable distance from the star for the temperatures to be right for the evolution of life. Believing that there are the inhabitable (that can be lived in) planets in our galaxy, most scientists agree that chances are that one or more of these planets support some life.
   However, many scientists wonder whether intelligent life exists on other planets. Some believe that thirty years of searching without any intelligent messages show that no one is out there. They say that the evolution of intelligence comparable to ours is unlikely.
   (44) Other scientists believe that our search hasn’t been long enough to rule out the possibility that intelligent life exists in our galaxy. Although our sun family 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 are much more advanced than ours. Perhaps these civilizations send 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.
According to the passage, how many planets in our galaxy might be inhabitable?
选项 A、5 billion
B、10 billion
C、15 billion
D、200 billion
答案A
解析
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