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  3. Earth’s Inner Core Scientists have long struggled to understand what lies at...

Earth’s Inner Core Scientists have long struggled to understand what lies at...

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问题 Earth’s Inner Core
Scientists have long struggled to understand what lies at the planet’s center. Direct observation of its center is impossible, so researchers must _______ (51) to other evidence.
In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves_______(52) jolts (晃动)from one side of Earth through the center to the other side. Then in 1936, Danish geophysicist Inge Lehmann studied the waves’________(53) to determine that within Earth’s core of molten (溶化了的)iron lies a solid inner core — but_______(54) that core was made of eluded (难倒)her. Other geophysicists quickly determined that Lehmann’s inner core was composed mostly _____(55) iron. Since then, Lehmann’s discovery has _______ (56) conventional Earth science.
But now scientists are challenging traditional theory with new and radical_______(57).For example, Earth’s center could actually contain an “inner core within the inner core”， claim Ishii and colleague Adam Dziewonski.
Analyzing hundreds of thousands of earthquake wave_______ (58), they maintain that the inner core has at its heart a tiny, even more solid sphere (球体).This sphere “may be the oldest fossil_______(59) from the formation of Earth,” says Dziewonski.
Dziewonski and Ishii speculate that shortly_______ (60) Earth formed around 4. 8 billion years ago, a giant asteroid (小行星)smashed into the young planet and nearly melted it. But Earth’s center didn’t quite melt; it_______(61) mass as the planet cooled.The core within a core may be the kernel (核心)that endured. “ Its presence could change our basic ideas about the ______ (62) of the planet/' Dziewonski says.
Dziewonski’s idea is tame (温和的)compared to the _______ (63) theories of independent geophysicist J. Marvin Herndon. Earth’s inner core is made not of iron, he claims, but a_______(64) of nickel and silicon. Herndon has a truly revolutionary notion：
Within the nickel silicide (桂化物)inner core is also an “inner” inner core — an 8 km-wide ball of the element uranium. Uranium is radioactive. Herndon thinks the uranium releases heat energy as its atoms_______(65) fission-split and crash into one another in a chain reaction.In other words,we may live on top of a gigantic, “natural”nuclear power plant.
选项 A after
B before
C since
D when
答案A
解析依据上下文，用after句子意思才讲得通。
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