G友帮帮看看新prep 256

jaze

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

有人能讲一下 at which这个时间状语修饰谁吗？ 是are known? 还是have made ？
另外为什么是are known？ 不是were known？ 划线部分到句末是什么意思？
谢谢

lonelyorchid

at which 修饰 date
古人会做石器，这件事情被现在人知道了。那只要就是永远都知道，不可能以前知道，现在不知道，所以一般现在时
pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.
将已知人类制作石器的年龄推早了150，000年。
正常的语序是：
pushing back the earliest date （when it is known that humans made stone tools） by more than 150,000 years .

jaze

at which 修饰 date
古人会做石器，这件事情被现在人知道了。那只要就是永远都知道，不可能以前知道，现在不知道，所以一般现在时
pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.
将已知人类制作石器的年龄推早了150，000年。
正常的语序是：
pushing back the earliest date （when it is known that humans made stone tools） by more than 150,000 years .

-- by 会员 lonelyorchid (2011/4/21 14:16:58)

the OA is C.  thanks all the same

cobra44

根据OG12 SC140的解释 "The idiomatic way of expressing estimation of age is with the phrase dated at", 就此推断凡是用date这个词推演年代的时候介词都用at, 也就是说which在这里修饰的是"the earliest date".

这句话的翻译应该是： 科学家将在埃塞俄比亚Afar地区的一个干枯河床细砂沉积物中发现的尖缘石片所属年代定为距今252-260万年，（该发现）将目前所知的人类最早制造石器的时间提前了至少15万年。

jaze

根据OG12 SC140的解释 "The idiomatic way of expressing estimation of age is with the phrase dated at", 就此推断凡是用date这个词推演年代的时候介词都用at, 也就是说which在这里修饰的是"the earliest date".

这句话的翻译应该是： 科学家将在埃塞俄比亚Afar地区的一个干枯河床细砂沉积物中发现的尖缘石片所属年代定为距今2520-2600万年，（该发现）将目前所知的人类最早制造石器的时间提前了至少15万年。

-- by 会员 cobra44 (2011/4/21 15:36:36)

But my questions are not met.  Thank you all the same

cobra44

不好意思看错了你的问题了，我觉得at which 修饰的是"humans are known to have made stone tools", 就是说在这个年代已经发现有人类制作石器了。
are known可能强调的是目前已知,  但是我觉得如果用were known从语法上应该是不错的。