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根据下面内容，回答题： 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.
According to the article, what causes a star to die? A．As its gases run out, it cools down.
B．It collides with other stars.
C．It can only live for about a million years.
D．As it gets hotter and hotter, it explodes.
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