求解一道SC题！

Chasinggirl

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.
A.when it is known that humans made
B.at which it is known that humans had made
C.at which humans are known to have made
D.that humans are known to be making      
E.of humans who were known to make



在prep2008语法笔记上看到这样一道SC题，至今还是想不懂A选项意思哪里不对了。。。有大神走过路过举手之劳帮个忙解答一下嘛？

啊喂我是蚊子

不是大神 一起讨论
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.
A.when it is known that humans made
明显这里的when想成为一个定语从句 修饰date
还原为：it is known that humans made stone tools at/其他介词 the earliest date我觉得没毛病
问题出在it is known？
不懂 求大神解答TUT
BTW: 答案选C吗？

practidealist

when it is known that  在这里意思是时间（我们知道that后面事实）被提前了150000年，而不是时间（原始人制造工具）被提前，逻辑上说不通

danyuchn

A沒有絕對的錯誤。是對上C選項而言的相對冗長。
在文法皆正確、語意合理的情況，C選項不使用虛詞，更為精簡。
如果這題沒有C，A選項可以是正確答案的。

cheer115

我看到这个题目也和楼主一样的困惑，A和C又什么区别吗？

Copy过来Ron大神的回答：
i think the real problem with the construction in choice (a) is that the clause following "when" is "it is known". in other words, that choice suggests that the fact is (was?) known at that date, an interpretation that clearly doesn't make sense in context.
in the correct answer, the "at which" is followed by a clause whose subject is "humans", and which describes the actual action that took place at that date. therefore, i think the idea is that this clause more accurately describes the chronology of the events: i.e., the toolmaking happened at that date, and the fact is known now.

lixiqi123

it is known that ,好像GMAT不是很喜欢。貌似无法翻译成众所周知