1道题看到3个答案，求牛人解答！

mistalpc

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 know to have made
   
   
4. that humans are known to be making
   
   
5. of humans who were known to make
   看到网上说答案是A或C，但XDF蓝皮书上的答案是E。到底应该是什么呢？
   

sdcar2010

Answer is 3 (or C).

The reason is as follows.
1) "the earliest dat when" and "the earliest date at which" are both acceptable English.  A very subtle difference between these two phrase is that the former is precise while the latter is kind of vague about the date.
2) Now let's analyze the clauses after the two phrases. "It is known that humans had made ... tool" means that on the date (not now) we KNEW that humans had made tools! Good heaven. Who says so! "Humans are known to have made" means that we know now that at that date human could make tools. This is what the sentence is about.

mistalpc

相当清楚！谢谢！