阅读理解。 Red Dwarf Red Dwarf stars (红矮星) can range in size from a hundred times smaller than the sun, to only a couple of times smaller. Because of their small size these stars burn their fuel very slowly, which allows them to live a very long time. Some red dwarf stars will live trillions of years before they run out of fuel. Then why are red dwarf stars red? Beacuse red dwarf stars only burn a little bit of fuel at a time, they are not very hot compared to other stars. Think of a fire. The coolest part of the fire at the top of the flame glows red, the hotter part in the middle glows yellow, and the hottest part near the fuel glows blue. Stars work the same way. Their temperatures determine what color they will be. Thus we can determine how hot a star is just by looking at its color. Like the Sun, these medium(中等的) -sized stars are yellow because they have a medium temperature. Their higher temperature causes them to burn their fuel faster.  This means they will not live as long, only about 10 billion years or so.  Near the end of their lives, these medium-sized stars swell up becoming very large.  When this happens to the Sun it will grow to engulf(吞 没) even the Earth.  Finally they shrink again, leaving behind most of their gas. This gas forms a beautiful cloud around the star called a Planetary Nebula(行星状星云) . When will the Sun expand into a giant, and then shrink leaving behind a planetary nebula? Don't worry. The sun is only about 5 billion years old.  It still has another 5 billion years before it will expand, and then turn into a planetary nebula. The Sun is so hot that when it dies, it will take a long time to cool off. The sun will die in about 5 billion years, but it will still glow for many billions of years after that. As it cools, it will be what is called a white dwarf star.  Finally, after billions maybe even trillions of years, it will stop glowing, at that point it will be what we call a black dwarf star. There are still no black dwarf stars in the Universe. 1. Why do the Red Dwarf stars burn their fuel very slowly? A. Because they are small. B. Because they don't move around. C. Because they are cool. D. Because they reflect light like the moon. 2. Which of the following best expresses the main idea of Paragra-ph 2? A. How red dwarf stars work. B. Why red dwarf stars burn a little fuel. C. Why red dwarf stars are red. D. How temperature determines the color. 3. Which of the following is closest in meaning to the underlined phrase' swell up'in Paragraph 3? A. Become smaller. B. Become larger. C. Keep the same. D. Grow high. 4. The title on the underline before Paragraph 3 is probably A. The Sun B. Yellow Stars C. Planetary Nebula D. Star Temperature 5. Which of the following websites may this passage be taken from? A. www. plant. com B. www. oceans. com C. www. inventor. com D. www. kidsastronomy. com