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未知题型 The major component of the 'pitch-black dust'(Line 5, Paragraph 3) should be ______D.carbon.

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未知题型 When stars like our Sun die, they bloat to become red giants and then eject gigantic clouds of gas and dust into spacE.Increasingly, however, scientists found themselves at a profound loss to explain how exactly dying stars could blow away these clouds. Now astrophysicists propose that unexpected chemical reactions during the formation of stardust could help solve this mystery.Stars smaller than the Sun and up to eight times as massive die by swelling up into red giants before shedding most of their mass to shrink into very compact, dying embers(灰烬) called white dwarfs. Two kinds of red giants exist-those with lots of carbon, and others richer in oxygen than in carbon. Carbon-rich stars release carbon particles during their death throes. Scientists proposed these pitch-black grains absorb rays from the dying star and get shot into space by starlight, a theory that fit both the observations and computer models.Increasingly, however, researchers could not explain how oxygen-rich stars like our own Sun could propel their clouds away during the final stages of mass loss. Oxygen-rich stars create large quantities of water vapor and silicates, such as quartz(石英砂) or sanD.These are transparent, meaning starlight should go right through them.Possible solutions have been contemplated by scientists. They first pondered whether the silicates might have iron in them, which would render them opaque instead of transparent. But calculations showed the dust grains would have evaporated if they had iron in them. They next wondered if enough molecules surrounded the core of a dying oxygen-rich star to block out its light and thus create a wind that blew the star's outermost layer into spacE.But their models suggested these molecules could not block off enough light and create strong enough Rinds.The scientists then suggested that pulsations occurred when stars die could perhaps force a star's matter out, but this idea did not match with astronomers' observations. But inspiration then dawned upon them. Perhaps some of the carbon in the oxygen-rich stars could help force the outer layers of the stars into spacE.They believe shock waves from the pulsations of dying stars could make carbon in oxygen-rich stars form. pitch-black dust.'The theory fits with all our subsequent model calculations, and it matches observations from dying oxygen-rich red giants'. Moreover', this mechanism strongly favors the presence of magnesium silicates over iron silicates in the interstellar medium', in agreement with recent findings from NASA's comet-sampling Stardust space probE.If proven correct, the beauty of the new scenario is that it suggests a common driving mechanism for many dying stars shedding their mass via dusty winds', with possible long-reaching consequences for the origin of chemical elements relevant for life'.By introducing the theory of red giant, the author intends to ______A.explain-the possible way of death of our Sun.B.show the two types of red giants.C.discuss on the way of red giants to become white dwarfs.D.explore the origin of lifE.

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