2. 外语
  3. The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. T

The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. T

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问题     The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. The Sun has existed in its present state for about 4 billion, 600 million years and is thousands of times larger than Earth.
    By studying other stars, astronomers can predict what the rest of the Sun’s life will be like. About 5 billon years from now, the core of the Sun will shrink and become hotter. The surface temperature will fall. The higher temperature of the center will increase the rate of thermonuclear reactions. The outer regions of the Sun will expand approximately 35 million miles, about the distance to mercury, which is the closest planet to the Sun. The Sun will then be a red giant star. Temperatures on the Earth will become too hot for life to exist.
    Once the Sun has used up its thermonuclear energy as a red giant, it will begin to shrink. After it shrinks to the size of the Earth, it will become a white dwarf star. The Sun may throw off huge amounts of gases in violent eruptions called nova explosions as it changes from a red giant to a white dwarf.
    After billions of years as a white dwarf, the Sun will have used up all its fuel and will have lost its heat. Such a state is called a black dwarf. After the Sun has become a black dwarf, the Earth will be dark and cold. If any atmosphere remains there, it will have frozen onto the Earth’s surface.
It can be inferred from the passage that the Sun______.
选项 A、is approximately halfway through its life as a yellow dwarf
B、has been in existence for 10 billion years
C、is rapidly changing in size and brightness
D、will continue as a yellow dwarf for another 10 billion years
答案B
解析 根据文章第一段第二句“The Sun has existed in its present state for about 4 billion，600 million years．和第二段第二句和最后一句About 5 billon years from now,the core of the sunwill shrink and become hotter．The sun will then be a red giant star．”可知，文章第一段说太阳作为一颗黄矮星已经存在了大约46亿年了。第二段又说大约50亿年后，太阳的中心部分将会收缩、变热。太阳将会变成巨大的火红的星球。那么就是说黄矮星的时代结束了。从而可知太阳以黄矮星的状态存在100亿年，那么以黄矮星的状态大约走过了其一半的生命历程。据此判断，答案是B。
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