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本来选A，仔细考虑后改支持C。做题时看不惯at which，稍微查了一下，the date when....和the date at which都行。大家都同意答案在A和C间产生，简化后比较一下：
A：(This finding) pushes back by X years the earliest date when it is known that humans made tools
C： (This finding) pushes 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.
[此贴子已经被作者于2005-3-23 13:42:12编辑过] | 
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发表于 2005-1-19 23:59:00
