单选题：Earth’s Inner Core Scientists have long struggled to underst

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（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 4.8billion 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 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 Hemdon 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．Hemdon thinks the uranium releases heat energy as its atoms_____（65）fission-split and crash into 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