PREP 08 - 256 请教！

cduserforgmat

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 have made

C.at which humans are known to have made

D.that humans are known to have made
E.of humans who were known to make
答案：C.
谁能给解释一下为什么不能用that修饰date, 解释是一个human are known to have made....的date呢？为什么一定要用at which?
谢谢各位！

cduserforgmat

up

mcs6436288

up还是没搞懂A为什么错了，有哪位DN解释一下啊。

Tiramisu_Xuan

Ron的解释：
i think the real problem with the construction in choice (a) is that the clause following "when" is "it is known". in other words, that choice suggests that the fact is (was?) known at that date, an interpretation that clearly doesn't make sense in context.人类用石头做工具是在150，000年前的时候知道的。。。逻辑意思不对
in the correct answer, the "at which" is followed by a clause whose subject is "humans", and which describes the actual action that took place at that date. therefore, i think the idea is that this clause more accurately describes the chronology of the events: i.e., the toolmaking happened at that date, and the fact is known now.

as for your other question, this isn't really a perfect tense, because it's actually not a tense at all -- it's an infinitive.
probably the easiest way to go here is to remember this as an idiomatic usage of the construction "known to". if the action is in the present, then you use "known to VERB"; if the action is in the past, then you use "known to have VERBed". as far as i know, these are the only two possible forms.