第6部分：完形填空
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。
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 （1） to other evidence.
In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves （2） 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' （3） to determine that within Earth's core of molten （熔化了的） iron lies a solid inner core － but （4） that core was made of eluded （难倒） her. Other geophysicists quickly determined that Lehmann's inner core was composed mostly （5） iron. Since then, Lehmann's discovery has （6） conventional Earth science.
But now scientists are challenging traditional theory with new and radical （7）. 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 （8）, 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 （9） from the formation of Earth," says Dziewonski.
Dziewonski and Ishii speculate that shortly （10） 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 （11） 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 （12） of the planet," Dziewonski says.
Dziewonski's idea is tame （温和的） compared to the （13） theories of independent geophysicist J. Marvin Herndon. Earth's inner core is made not of iron, he claims, but a （14） of nickel and silicon. Herndon has a truly revolutionary notion: Within the nickel silicide （硅化物） 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 （15） 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.
