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.

51 A try B leave C turn D point

52 A create B receive C feel D overcome

53 A work B solution C job D patterns

54 A whether B what C why D how

55 A from B within C of D to

56 A followed B dominated C restored D opposed

57 A ideas B demands C phenomena D movements

58 A things B acts C methods D records

59 A taken B benefited C left D kept

60 A after B before C since D when

61 A expanded B modified C gained D melted

62 A size B origin C structure D shape

63 A radical B traditional C classical D conventional

64 A system B copy C model D compound

65 A charge B last C experience D show