请问如何判定 it 在句中是做形式主语（宾语）？还是代词？
比如：
# Prep2008-256：
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
答案：C
A,B的错误之一在于：“此选项表达的意思是A fact is known at the earliest date，逻辑含义错误。”
因此我得到的结论是，It 在这道题里做的是代词，错误的指代了the fact。
而我以为它是形式主语，真正主语是 humans。为什么不对呢？
而在#PREP2008-241的补充说明中有举例：
it可以替代句中的不定式或从句等, 充当形式主语或形式宾语。one与that均无此用法。
It is known to everybody that the moon travels around the earth once every month.
所以彻底晕了。。。。
