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  3. It is just about impossible to imagine how big the universe is. All we can do is to describe certain things we know about it and

It is just about impossible to imagine how big the universe is. All we can do is to describe certain things we know about it and

admin2019-04-07  56
问题
    It is just about impossible to imagine how big the universe is. All we can do is to describe certain things we know about it and hope that these give an idea of the scale of the cosmos.
    On clear nights, in places where light pollution from cities does not make it too difficult to see the stars, we can see many points of lights in the sky which are like a white cloud of stars. This is our home galaxy, the Milky Way. There are probably as many or more galaxies in the universe as there are stars in the Milky Way. Some estimates say there are 100 billion galaxies, each with millions of stars. Our home galaxy, like all others, is still forming stars all the time from gas floating in space.
    Most galaxies contain not only stars, but also black holes. These are the tiny remains of stars that have collapsed in spectacular explosions. Gravity is so intense in black holes that light bends, so it can’t escape to be seen from outside. The mass of a black hole is so concentrated that the Earth would be reduced to the size of a tennis ball, while remaining as heavy as it is now.
    Our nearest neighboring galaxy is Andromeda, which is two million light years away. A light year is the distance covered by light during one year, traveling at 300, 000 kilometers a second. The most distant galaxies that we can see, even with telescopes, are 10 to 12 billion light years a-way. That means it has taken the light from those galaxies that amount of time to reach the Earth. When that light left those galaxies, the universe was still quite young. The universe began with a gigantic explosion, called the Big Bang. About 14 to 15 billion years ago, something about the size of an atom suddenly started expanding dramatically. Out near the edge of the universe, there is still Cosmic Microwave Background, which is the heat left over from the Big Bang. Today the universe is still expanding in a process called inflation. And it may go on accelerating outwards from the original point for ever. The latest theories show that only a tiny proportion of the universe is made up of the kind of matter that we can see: about 15% of it is something called dark matter, and the rest, about 75% , is dark energy, which is driving the inflation of the universe.
The distance covered by light during a year is called______.
选项
答案a light year
解析 (文章最后一段介绍一光年就是光行走一年的路程。原句为A light year is the distancecovered by light during one year，因此该空应填a light year。)
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