根据下列文章,请回答 51～65 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 ________ (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 around4.8 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. 第 51 题