争议题..

karma3001

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
肿么就不能选c呢  是不是有条规则介词+which优于when where

aeoluseros

should be C. details refers to prep08 grammar notes(prep1-256).

阿汤

不是。。 关系词

引导非限定性定语从句的关系代词：
　　
指代对象    指代人    指代物
主格    who    which,as
宾格    whom    which,as
所有格    of whom, whose    which, of which, whose
　
引导非限定性定语从句的关系副词主要有：when,where

介词+which、when where 在这里没有差。which在定语从句中要充当成分。

C答案 humans are known   有一个被动在里面。人类被知道制造石器。排除

karma3001

to 2L A里不也存在被动吗?

阿汤

it is known that humans made stone tools.
这个句子是这样的。it是形式主语真正主语是that 后面的从句，也就是"humans made stone tools" is known  这样是可以的。