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  3. How many things can you see in the night sky? A lot! On a clear night you might see the Moon, some planets, and thousands of spa

How many things can you see in the night sky? A lot! On a clear night you might see the Moon, some planets, and thousands of spa

admin2012-12-31  86
问题     How many things can you see in the night sky? A lot! On a clear night you might see the Moon, some planets, and thousands of sparkling stars. You can see even more with a telescope.
    But scientists believe there are some things in the sky that we will never see. We won’t see them with the biggest telescope in the world, on the clearest night of the year. That’s because they’re invisible. They’re the mysterious dead stars called black holes.
    As a star’s gases burn, they give off light and heat.  But when the gas runs out, the star stops burning and begins to die.
    As the star cools, the outer layers of the star pull in toward the center. The star squashes (挤压) into a smaller and smaller ball. If the star was very small, the star ends up as a cold, dark ball called a black dwarf. If the star was very big, it keeps squashing inward until it’s packed together tighter than anything in the universe.
    Imagine if the Earth were crushed until it was the size of a tiny marble. That’s how tightly this dead star, a black hole, is packed. What pulls the star in toward its center with such power? It’s the same force that pulls you down when you jump--the force called gravity. A black hole is so tightly packed that its gravity sucks in everything--even light. The light from a black hole can never come back to your eyes. That’s why you see nothing but blackness.
    So remember: Scattered in the silent darkness are black holes--the great mystery of space.
Why can’t you see light when you look at a black hole?
选项 A、Because most black holes are so far away.
B、Because the gravity of a black hole is so strong that it sucks? the light inward.
C、Because as the star’s gases burn, it stops giving off heat and light.
D、Because as a star cools, its outer layers pull in toward its center.
答案B
解析 倒数第二段。在星星燃料烧尽冷却的过程中，自身的引力把所有的东西往里吸，包括光，而不是释放。所以你看不见光。选项B是正确的。选项C与事实相反。选项D不完整，除了外层被吸入，最关键的是光也被吸入。
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