The onset of the so-called Little Ice Age ,which followed the Medieval Warm Period of the eighth through tenth centuries,may represent the most recent time that the oceansdynamic changed in this way. If ongoing climate-history studies support Bondshypothesis of 1,500-year cycles, scientists may establish a major natural rhythmin Earths temperatures that could then be extrapolated into the future.Because the midpoint of the Medieval Warm Period was about A.D. 850, anextension of Bonds cycles would place the midpoint of the next warm intervalin the twenty-fourth century.

　　21.1. According to the passage, which of the following istrue of the rock fragments contained in the sediments studied by Bond?

　　A. The majority of them are composed of red sandstone.

　　B. They must have reached their present location over 1,500 years ago.

　　C. They were carried byicebergs to their present location.

　　D. Most of them were carried to their present location during a warm period inEarths climatic history.

　　E. They are unlikely to have been carried to their present location during theLittle Ice Age.

　　21.2. In the final paragraph of the passage ,the author is concerned primarily with

　　A. answering a question about Earths climatic history

　　B. pointing out a potential flaw in Bonds hypothesis

　　C. suggesting a new focus for the study of ocean sediments

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