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