A、A white planet. B、A red giant star. C、A new big planet. D、A black dwarf star. D题干询问当太阳上层膨胀时太阳核最终会变为什么。对话中男士说当太阳的上层膨胀并失去物质时，太阳核

W: Wow! What a beautiful sunset.
M: Enjoy it while it lasts, the sun won’t be around forever.
W: You mean, one day the sun will disappear?
M: Not exactly disappear, but the sun will eventually burn out and die.
W: Won’t that make things really cold down here?
M: Actually, there won’t be any here. (19) When the sun dies, the earth will be destroyed too.
W: Wow, hold on, start from the beginning.
M: Okay. Like all stars, (20)the sun is basically a giant nuclear reactor that bums hydrogen to create energy. But like any reactor, the sun needs fuel to keep going. It’s been going strong for around 4. 5 billion years, and will probably keep burning for about five billion more.
W: But then, the fuel will run out?
M: Exactly. When it does, gravity will cause the sun’s core to contract. (21) When it contracts the core will get hotter, which will heat up the sun’s upper layers and make them expand. The sun will then become what’s called a red giant, and become bigger W: It is bad for us.
M: After a few billion years the core will begin to cool and expand. (22) The sun’s upper layers will expand and lose material, and eventually the core will cool enough to become a white dwarf star, and finally a black dwarf.
W: So, we’re pretty much doomed.
M: Yes, but not for billions of years. Who knows? By then, human might have been living on another planet.
19. What may happen to the earth when the sun dies?
20. How can the sun create energy?
21. What will happen to the sun’s core when it contracts?
22. What will the core finally be after the sun’s upper layer expanded?