Scientists have long struggled to understand what lies at the planets center, Direct

　　observation of its center is impossible，so researchers must_____to other evidence.

　　In 1889，a German scientist detected a severe earthquake in Japan.Geophysicists concluded

　　that shock waves_____joltsfrom one side of Earth through the center to the

　　other side.Then in 1936，Danish geophysicist Inge Lehmann studied the waves______

　　to determine that within Earths core of molteniron lies a solid inner core-but_____

　　that core was made of eludedher.Other geophysicists quickly

　　determined that Lehmanns inner core was composed mostly_____iron. Since then，

　　Lehmanns discovery has_____conventional Earth science.

　　But now scientists are challenging traditional theory with new and radical _____

　　For example，Earths center could actually contain aninner core within the inner core，claim Ishii and colleague Adam Dziewonski.

　　Analyzing hundreds of thousands of earthquake wave _____,they maintain that the inner core has at its heart a tiny, even more solid sphere . This spheremay be the oldest fossil____from the formation of Earth，says Dziewonski.

　　Dziewonski and Ishii speculate that shortly_____ Earth formed around 4.8billion years ago，a giant asteroidsmashed into the young planet and nearly melted it. But Earths center didnt quite melt;it______mass as the planet cooled. The core within a core may be the kernelthat endured.Its presence could change our basic ideas about the_____ of the planet，Dziewonski says idea is tamecompared to the_____theories of independent geophysicist J.Marvin Herndon.Earths inner core is made not of iron，he claims

　　but a_____of nickel and silicon Hemdon has a truly revolutionary notion：Within the nickel silicideinner core is also aninnerinner corean 8 kmwide ball of the element uranium.Uranium is radioactive.Hemdon thinks the uranium releases heat energy as its atoms_____fission-split and crash into another in a chain reaction. In other words,we may live on top of a gigantic, naturalnuclear 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

　　Scientists have long struggled to understand what lies at the planets center, Direct

　　observation of its center is impossible，so researchers must_____to other evidence.

　　In 1889，a German scientist detected a severe earthquake in Japan.Geophysicists concluded

　　that shock waves_____joltsfrom one side of Earth through the center to the

　　other side.Then in 1936，Danish geophysicist Inge Lehmann studied the waves______

　　to determine that within Earths core of molteniron lies a solid inner core-but_____

　　that core was made of eludedher.Other geophysicists quickly

　　determined that Lehmanns inner core was composed mostly_____iron. Since then，

　　Lehmanns discovery has_____conventional Earth science.

　　But now scientists are challenging traditional theory with new and radical _____

　　For example，Earths center could actually contain aninner core within the inner core，claim Ishii and colleague Adam Dziewonski.

　　Analyzing hundreds of thousands of earthquake wave _____,they maintain that the inner core has at its heart a tiny, even more solid sphere . This spheremay be the oldest fossil____from the formation of Earth，says Dziewonski.

　　Dziewonski and Ishii speculate that shortly_____ Earth formed around 4.8billion years ago，a giant asteroidsmashed into the young planet and nearly melted it. But Earths center didnt quite melt;it______mass as the planet cooled. The core within a core may be the kernelthat endured.Its presence could change our basic ideas about the_____ of the planet，Dziewonski says idea is tamecompared to the_____theories of independent geophysicist J.Marvin Herndon.Earths inner core is made not of iron，he claims

　　but a_____of nickel and silicon Hemdon has a truly revolutionary notion：Within the nickel silicideinner core is also aninnerinner corean 8 kmwide ball of the element uranium.Uranium is radioactive.Hemdon thinks the uranium releases heat energy as its atoms_____fission-split and crash into another in a chain reaction. In other words,we may live on top of a gigantic, naturalnuclear 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