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 at which humans are known to have made stone tools.
语法 Sentence Correction-1763
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sonic 2014-08-15 16:07:18
這題我做錯。 邏輯： [be known to]忽略點：A be known to B，若B時間早於A，則使用現在完成式。所以刪去DE [V-ing禁忌]忽略點→V-ing切忌被作為主要修飾的「名詞」，分裂句乃強調，但強調者為名詞(且是主要關鍵名詞)，所以不可以，刪去AB
littlesouthsea 2015-12-03 20:28:16
when和at which都可以。 当be known 和 do sth的发生的时间一致时，用be known to do sth； 而do sth发生在be known之前时，则要用be known to have done sth。
Susie_yan 2015-10-08 15:54:30
发生在know之前 用完成时
许栩 2016-11-09 16:52:29
at which = at the date
queenkat 2016-10-24 20:34:42
逻辑意思得考察
Goooaway 2016-06-11 14:06:00
https://www.manhattanprep.com/gmat/forums/scientists-have-dated-sharp-edged-flakes-of-stone-found-in-t9546.html 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 for
Goooaway 2016-06-10 20:16:11
be known to have done sth：当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth
ocean09960802 2016-05-25 19:10:49
1) be known to have done sth的问题：当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth。 大全748：...few people are known to have recovered from the disease once the clinical symptoms have appeared. 大全849：...few people have been known to taste it for the first time without requesting more. og49：......satellites now known to orbit the distant planet. *：事实上在看这两个结构时，可以把be known to这个类似助动词的成分去掉；由于跟在be known to后面的动词要用原型，所以去掉be known to以后就可以单纯地比较have done和do，have done所表示的发生时间比do所表示的发生时间要早，如果事件发生在过去就用have done，发生在现在就用do。同时如果be known to do/have done sth后面接状语那么修饰的是do/have done，而不是known. 2) date to/at区分 1) date….to 可追溯sth到...几年，强调发生的时间点，后面是ago 2) date….at 年代判定、鉴别/年份有….年之久，强调发生到现在经过的时间有多长，后面是old
zhangjunya 2016-03-10 20:55:31
当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth。当人们被知道在。。。
huyingchuan 2016-02-20 15:53:33
1) be known to have done sth的问题：当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth。 *：事实上在看这两个结构时，可以把be known to这个类似助动词的成分去掉；由于跟在be known to后面的动词要用原型，所以去掉be known to以后就可以单纯地比较have done和do，have done所表示的发生时间比do所表示的发生时间要早，如果事件发生在过去就用have done，发生在现在就用do。同时如果be known to do/have done sth后面接状语那么修饰的是do/have done，而不是known.
xiaobaitu 2016-02-19 15:19:13
1) be known to have done sth的问题：当be known 和 do sth的发生的时间一致时，用be known to do sth；而do sth发生在be known之前时，则要用be known to have done sth。 大全748：...few people are known to have recovered from the disease once the clinical symptoms have appeared. 大全849：...few people have been known to taste it for the first time without requesting more. og49：......satellites now known to orbit the distant planet. *：事实上在看这两个结构时，可以把be known to这个类似助动词的成分去掉；由于跟在be known to后面的动词要用原型，所以去掉be known to以后就可以单纯地比较have done和do，have done所表示的发生时间比do所表示的发生时间要早，如果事件发生在过去就用have done，发生在现在就用do。同时如果be known to do/have done sth后面接状语那么修饰的是do/have done，而不是known.
vickiaro 2015-11-22 22:20:54
是 sb is known to do sth.。
