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[填空题]Leonardo da Vinci was the first man to suggest that growing trees add a ring in their trunks each year. The (47) is these rings relates to the physical conditions which the tree experiences. Thus, trees grown in a (48) area and time each develop a pattern or configuration of their rings.
This (49) was of little significance until Andrew E. Douglas began to study tree rings in Arizona in 1600. Using a technique called cross dating, he was able to employ tree rings to the study of archaeological sites and date the ruins with (50) . Some were as old as 6700 B.C. ! This study of tree rings is called dendrochronology(树木年代学).
In time the cross dating was (51) by a carbon 14 process. This approach measured the amount of carbon 14 radiating from a piece of wood and (52) determined the age of that wood. Further use of the carbon 14 technique has shown that the radiation process is more complex and less (53)

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