单项选择题
______ in large part by the cold, dry climate, the fragment, which includes a canine tooth, still held some genetic remnants.
A．Preserving
B．Having been preserved
C．Having preserved
D．Which was preserved
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单项选择题
31().
A．classes
B．courses
C．programs
D．plans
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单项选择题
If computers ______, many problems on space flight could not have been solved.
A．had not been invented
B．are not invented
C．were not invented
D．have not invented
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