再请教T-4-Q1 TS-7-41

flywer

搜了讨论帖 但是依然不明白A和C的区别

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

以前讨论的都说是A和C的句意是有区别的，我也觉得有，但就是说不清楚在哪里。还有那个完成时are known to have made用在这里是什么意思?
请大家帮忙解答一下
谢谢！

flywer

终于找到了一个前人贴中我觉得可以接受的答案   也贴出来跟大家一起分享下

A：(This finding) pushing back by X years the earliest date when it is known that humans made tools
C： (This finding) pushing back by X years the earliest date at which humans are known to have made tools.
1. A最大的问题是歧义。when... 本应作定语，但也是个完美的状语从句，意思就变成：当知道人类造工具这件事情的时候，this finding pushes back....。同一个东西可能作两种成分，并导致两个不同的意思，这种表达方式在GMAT中是禁止的。B 就没这个问题，因为at which...只能作定语从句，不能作状语。
2. be known to have done sth的问题。我觉得，当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth。这里的情形显然属于后者。
所以选C
附known to do sth的例句：
1. 大全748：...few people are known to have recovered from the disease once the clinical symptoms have appeared.
2. 大全849：...few people have been known to taste it for the first time without requesting more.
3. og49：......satellites now known to orbit the distant planet.