标题: 求助GWD一道语法题。感激不尽啊 [打印本页] 作者: wjk 时间: 2011-4-8 18:37 标题: 求助GWD一道语法题。感激不尽啊 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A 我想请教一下为什么不选B？发生在过去的过去不应该用过去完成时么？而且我记得date后面应该是跟to或者at的吧？ 谢谢！