单选题：Earth's Inner Core　 　Scientists have" long struggled to unde

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 scientistdetected 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 (熔化了的)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 B．e 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(硅化物) inner core is also.an "inner" inner core -- an 8 kmowide 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. A．try
B．leave
C．turn
D．point