[求助]牛牛们帮偶下吧

1.  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

E.of humans who were known to make

为什么是E呢   of humans who不是把重心转移到人上了？ 难道没有it is known that的表达方法吗？

非NN，个人看法：

有 it is known that的用法，it作形式主语，that从句是真正主语，所以可以变成这样：that-clause is known.再看这个题，把A选项换一下，就发现问题了：that humans made stone tools is known when， when指代the earliest date。句意变成了：在那时就知道humans made stone tools，不符合逻辑。

open to discuss.....