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.
题目释义：Scientists have dated A found in B to # years ago, pushing back by # years the earliest date at which humans are known to have made.
A. 关系代词用when和at which都是可以的；此选项表达的意思是A fact is known at the earliest date，逻辑含义错误。
B. 用形式主语+is known+主语从句累赘，不如are known to have made简洁；主语从句的时态错误，用过去完成时需要句子逻辑上明确表明过去的过去。
D. 定语从句关系代词错误；are known to be doing sth.用法unidiomatic，要表示正在进行也只要are known to do即可。
E. date of + noun用法错误，其后面那个名词都是在某个时间出现的事件，如：date of birth、date of departure、date of marriage，而date of humans不是什么事件，所以不能说 date of；应该用are known to have made，因为known是发生在现在，make发生在known之前。
( ¨̮ )
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.
this is one of those problems from which you can learn about the ways in which these sorts of constructions are used. indeed, it is precisely from these sorts of problems that you must learn about the niceties of these constructions, as the gmat's usage can, and does, sometimes vary from that of other sources. in this sort of construction, i'm pretty sure that either "when" or "at which" would be acceptable. (clearly, the latter is acceptable -- it appears in the correct answer, after all -- but i wouldn't object to the former.) if you want to get really subtle, i think (not 100% sure) that "the date when" is used for actual, precise calendar dates, whereas "the date at which" is used for the usually more vague dates of historical events, such as the one in this problem. but i'm sure the test is not going to depend on this sort of nuanced difference. 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. 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. --曼哈顿解释