题目内容：
根据材料，回答问题。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.Geophysicistsconcluded that shock waves (52) _________ jolts (晃动) from one side of Earth through the centerto 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 quicklydetermined 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)_________. Forexample, Earth's center could actually contain an "inner core within the inner core, claim Ishiiand 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," saysDziewonski.
Dziewonski and Ishii speculate that shortly(60)_________ earth formed around 4.8billionyears ago, a giant asteroid (小行星) smashed into the young planet and nearly melted it. ButEarth's center didn't quite melt; it(61) _________ mass as the planet cooled. The core within acore may be the kernel (核心) that endured. "Its presence could change our basic ideas aboutthe(62) _________ of the planet, " Dziewonski says idea is tame (温和的) compared to the(63)_________ theories of independent geophysicist J. Marvin Herndon. Earth's inner core ismade not of iron, he claims but a(64)_________ of nickel and silicon Hemdon has a trulyrevolutionary notion: Within the nickel silicide (硅化物) inner core is also an "inner" innercorean 8 km -- wide ball of the element uranium. Uranium is radioactive. Hemdon thinksthe uranium releases heat energy as its atoms(65)_________ fission-split and crash into another in achain reaction. In other words, we may live on top of a gigantiC． "natural" nuclear powerplant.
请在第__(51)__处填上正确答案。 A．try
B．leave
C．turn
D．point
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