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dodomerry

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.

1. when it is known that humans made
   
2. at which it is known that humans had made
   
3. at which humans are known to have made
   
4. that humans are known to be making
   
5. of humans who were known to make
   
   答案是A，请问如何翻译？各选项如何排除？
   

dodomerry

谢谢~
请问C的are为什么时态错误？humans are known to如果解释为现在人们都知道那时的人类制作石头器具，现在时就可以成立呀...

andycooler

the earliest date when it is known that humans made stone tools.
可以翻译为众所周知人类使用石器的最早的年代。

D,E可以立刻排除，逻辑意思不正确。
when 和 at which 都能够使用。但C中are 时态错误。B中也没有明显表示过去完成时的意思。