第3部分：概括大意与完成句子(第23~30题，每题1分，共8分)

　　下面的短文后有2项测试任务：(1)第23~26题要求从所给的6个选项中为指定段落每段选择1个小标题;(2)第27~30题要求从所给的6个选项中为每个句子确定一个最佳选项。

Black Holes

      1　Black holes can be best described as a sort of vacuum, sucking up everything in space. Scientists have discovered that black holes come from an explosion of huge stars. Stars that are near death can no longer burn due to loss of fuel, and because its temperature can no longer control the gravitational（重力的）force, hydrogen ends up putting pressure onto the star’s surface until it suddenly explodes then collapses.
      2　Black holes come from stars that are made of hydrogen, other gases and a few metals. When these explode it can turn into a stellar-mass（恒星质量）black hole, which can only occur if the star is large enough (should be bigger than the sun) for the explosion to break it into pieces, and the gravity starts to compact every piece into the tiniest particle. Try to see and compare: if a star that’s ten times the size of the sun ends up being a black hole that’s no longer than 70 kilometers, then the Earth would become a black hole that’s only a fraction of an inch!
      3　Objects that get sucked in a black hole will always remain there, never to break free. But remember that black holes can only gobble up（吞噬）objects within a specific distance to it. It’s possible for a large star near the sun to become a black hole, but the sun will continue to stay in place. Orbits do not change because the newly formed black hole contains exactly the same amount of mass as when it was a star, only this time its mass is totally contracted that it can end up as no bigger than a state.
      4　So far, astronomers have figured out that black holes exist because of Albert Einstein’s theory of relativity. In the end, through numerous studies, they have discovered that black holes truly exist. Since black holes trap light and do not give off light, it is nearly impossible to detect black holes via a telescope. But astronomers continue to study galaxies, space and the solar system to understand how black holes might evolve. It is possible that black holes can exist for millions of years, and later contribute to a bigger process in galaxies, which can eventually lead to creation of new entities. Scientists also credit black holes as helpful in learning how galaxies began to form.

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