A、5 billion years. B、4 billion years. C、4.5 billion years. D、5.4 billion years. C原义直现。大阳的强势期已有四十五亿年，可在对话中直接找到答案。

W: Wow, what a beautiful sunset.
M: Enjoy it while it lasts. The sun won’t be around forever, you know.
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.  When the sun dies, the earth will be destroyed too.
W: Whoa, hold on. Start from the beginning.
M: Ok. Like all stars the sun is basically a giant nuclear reactor that burns 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. And when it does, gravity will cause the sun’s core to contract. 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 its radius will reach just past the orbit of Venus.
W: Which is bad for us.
M: We’ll be burned to a crisp.
W: Sounds painful.
M: After a few billion years, the core will begin to cool and expand. 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 humans might be living on another planet. Or we’ll have been enslaved by a bunch of dirty apes!
W: I think someone’s been watching too much cable TV.