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标题: 请教一题语法GWD6-41 [打印本页]

作者: shopping_han 时间: 2008-9-10 17:34
标题: 请教一题语法GWD6-41

Scientists have dated sharp-edged flakes of stone found in the fine-grained sediments of a dry riverbed in the Afar region of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.

when it is known that humans made
at which it is known that humans had made
at which humans are known to have made
that humans are known to be making
of humans who were known to make

答案是a，，b和c不能选的理由是at which应该是on which呢还是选项的时态错误？

多谢请指教！

作者: dianecarton 时间: 2008-9-10 19:22

同问

[此贴子已经被作者于2008-9-11 22:03:22编辑过]

作者: shopping_han 时间: 2008-9-10 19:47
那就是说不选B，C的理由是因为时态问题咯？

作者: dianecarton 时间: 2008-9-10 21:10

迷惑

[此贴子已经被作者于2008-9-11 22:03:03编辑过]

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