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(C)
E.of humans who were known to make
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.
选项分析：
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.at which it is known表达的意思是A fact is kn
own at the earliest date，逻辑含义错误；主语从句的时态用过去完成时不够合理。C.Correct；the earliest date at which humans are known to have made stone tools.=humans are known to have made stone tools at that date (the earliest date) – AlienX.
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。
A项中， “when it is known that humans made”难道不可以理解成it is...that是强调么？换回来就是when humans are known to made? A项的原解释实在看不懂啊
-- by 会员 风行稚 (2012/10/18 19:59:09)
