新PREP1-256，求助

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

这是一道gwd争议题呀~不过我忘了是哪一套上的题。。。

其实，由于修饰的是date，所以很容易排除A、D、E项，剩下B和C，排除B，罗嗦！“被知道人们已经开始使用stone tools的最早的日期”！用完成式完全解释的清啊！

1.这道题需要一个定语从句修饰date，

一般情况下我们用，the date (when/at which/that)+什么事情发生/什么人做了什么

这里中间加上"it is known that"意思上就不通顺了,因为"it is known that"这种句型本身不是一个用来改定语从句的好材料。

A和B就排除了。

至于是when/at which/that，我拿不准是不是gmat有特别的要求，感觉一般都可以用的。

D的话，这里没有正在进行的意思，所以making是不对的。

E扭曲了意思，这里想说的是人类制造石器这件事情的时间，所以date的定语应该是一件事情，而不是humans。

用完成时"to have made"表示一种动作发生在过去的意思。比如：

A number of hurricanes are known to have impacted on Tuvalu during the nineteenth and twentieth centuries.

Mongoloid descent and are known to have created an advanced civilization in the highlands of the Altai Mountains between 600 and 300 BC.

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

原句的earliest最高级修饰date强调时间点，所以使用at which由于when；A，B选项逻辑意思严重违背原句意思，A，B选项的意思是humans made stone tools被it is known的时间被push back...；原句是想说human are known to have made stone tools的时间被pushing back；

过去已经发生的事情，现在被发现也是现在完成时使用的情况之一。

原句的earliest最高级修饰date强调时间点，所以使用at which由于when；A，B选项逻辑意思严重违背原句意思，A，B选项的意思是humans made stone tools被it is known的时间被push back...；原句是想说human are known to have made stone tools的时间被pushing back；

过去已经发生的事情，现在被发现也是现在完成时使用的情况之一。

虽然知道别的选选项有更错的地方

但C的时态真是让我纠结半天，总感觉要么用过去时，要么用过去完成时。

干嘛要用现在完成呢？希望NN解答下！！