The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its centerthat convert hydrogen to helium. The Sun has existed in its present state for about 4 billion, 600million 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 surfacetemperature will fall. The higher temperature of the center will increase the rate of thermonuclearreactions. The outer regions of the Sun will expand approximately 35 million miles, about thedistance 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 itshrinks to the size of the Earth, it will become a white dwarf star. The Sun may throw off huge amountsof 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 havelost its heat. Such a state is called a black dwarf. After the Sun has become a black dwarf, the Earthwill be dark and cold. If any atmosphere remains there, it will have frozen onto the Earth's surface.
单选题
What is the primary purpose of this passage? A. To alert people to the dangers posed by the Sun. B. To discuss conditions on Earth in the far future. C. To present a theory about red giant stars. D. To describe changes that the Sun will go through.
【正确答案】
D
【答案解析】[解析] 纵观全文，可以看出是预测未来太阳要经历的变化。据此判断，答案是D。
单选题
It can be inferred from the passage that the Sun______ 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
【正确答案】
B
【答案解析】[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 sun will 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。
单选题
When the Sun becomes a red giant, what will conditions be like on Earth? A. Its atmosphere will freeze and become solid. B. It will be enveloped in the expanding surface of the sun. C. It will become too hot for life to exist. D. It will be nearly destroyed by nova explosions.
【正确答案】
D
【答案解析】[解析] 根据文章第三段第三句“such as when they are asked to offer a toast at a wedding reception or to participate in a televised debate or interview．”可知，太阳将会变成火红的星球时。 地球上的生命，由于温度过高因此无法继续。据此判断，答案是D。
单选题
According to the passage, which of the following best describes the sequence of stages that theSun will probably pass through? A. Yellow dwarf, white dwarf, red giant, black giant B. Red giant, white dwarf, red dwarf, nova explosion C. Yellow dwarf, red giant, white dwarf, black dwarf D. White dwarf, red giant, black dwarf, yellow dwarf