题目解析：
本题为细节题，题目问：第五段中关于太阳死亡的描述，以下哪一项是正确的？我们逐一排除错误选项。
A选项说太阳的外层会剧烈膨胀，这是由外层温度下降导致的。但原句是这么说的“The outer envelope will then grow rapidly, increasing, in a short interval, hundreds of times in size. As it expands, it will cool down by thousands of degrees.”从中可知， “温度下降”是伴随着“剧烈膨胀”而出现的现象，而不是因为温度下降才导致的膨胀，不存在这种因果关系，故A排除。
B选项说太阳会先变成红巨星，再过了几百万年之后它会变成白矮星。文中说红巨星会“It will exist that way for millions of years”以这种方式存在数百万年，然后变成白矮星，再逐渐死亡。所以B选项符合原文，正确。
C选项说当太阳把外部的物质都释放到宇宙中后，它的内核在一段时间内会继续发生核聚变。但原文说“The core will shrink, giving off the last of its energy, and the Sun will finally die.”内核会发生收缩，而不是核聚变。所以C选项不符合原文，排除。
D选项说太阳外层在膨胀的时候，它会把所有的物质都释放到宇宙中。但是文中说“It（红巨星）will exist that way for millions of years, gradually releasing the material of its outer envelope into space.”所以逐渐释放完外层物质是在红巨星形成之后的几百万年时间里，而不是在太阳外层膨胀还未最终形成红巨星的时候。而且，太阳释放了外层物质，但还保留了内核部分。所以 D选项不符合原文，排除。
Once the Sun passes into its "postnuclear" phase, it will separate effectively into two different regions: an inner zone and an outer zone. While no more hydrogen fuel will remain in the inner zone, there will be a small amount left in the outer zone. Rapidly, changes will begin to take place that will serve to tear the Sun apart. The inner zone, its nuclear fires no longer burning, will begin to collapse under the influence of its own weight and will contract into a tiny hot core, dense and dim. An opposite fate will await the outer region, a loosely held-together ball of gas. A shock wave caused by the inner zone's contraction will send ripples through the dying star, pushing the stellar exterior's material farther and farther outward. The outer envelope will then grow rapidly, increasing, in a short interval, hundreds of times in size. As it expands, it will cool down by thousands of degrees. Eventually, the Sun will become a red giant star, cool and bright. It will be so large that it will occupy the whole space that used to be the Earth's orbit and so brilliant that it would be able to be seen with the naked eye thousands of light-years away. It will exist that way for millions of years, gradually releasing the material of its outer envelope into space. Finally, nothing will be left of the gaseous exterior of the Sun; all that will remain will be the hot, white core. The Sun will have become a white dwarf star. The core will shrink, giving off the last of its energy, and the Sun will finally die.
