求助GWD一道语法题。感激不尽啊

wjk

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A
   我想请教一下为什么不选B？发生在过去的过去不应该用过去完成时么？而且我记得date后面应该是跟to或者at的吧？
   谢谢！
   

hellcan

貌似这个date表示时间。。。when从句修饰date。。。

76063030

嗯~我记得我们老师是这样说的，一般过去完成时的句子应该有一个模糊的时间概念，如by/before+时间点。而这道题的date是具体的时间点，只能用一般过去时。至于date at 什么的固定搭配在用了where后就不存在介词了啊，这就是where 和which的用法区别啊~

76063030

打错了，应该是when和which的区别……

lepus0107

这里没有过去的过去，就是一般过去啊……when和at which在这里都可以吧，不是重点。