Question

it is known to us all that the old scientists, for whom life was hard in the past

(it) is known to us all that the old scientists, for (whom) life was hard in the past ,still works hard in his eighties.A.it,whom  Bas,whom  C.as,whose   Dwhat  whom 为什么选A，我觉得B也蛮对的

Answer 1

第二个空填WHOM，因为是在介词后面用宾格 ，这个你掌握得很好。
至于第一个空，只能用IT，因为该句是一个主语从句，也就是说，IT代替了THAT后面的从句作形式主语。
如果想用AS，应该是这样，As is known to ua all, the old scientist.....
也就是说不能用that, 还得加一个逗号，AS这一部分用作插入语
希望对你能有帮助。

Answer 2

It is known to us all that the old scientists, for (whom) life was hard in the past ,still works hard in his eighties.
As is known to us all, the old scientists, for (whom) life was hard in the past ,still works hard in his eighties.
注意上两句的不同,
It is known to us all that ...that 前没有标点.也就是说，it代替了that后面的从句作形式主语。
As is known to us all,有标点逗号,因为As在这儿引导非限定性定语从句. As 指代下文那句话.

Answer 3

(it) is known to us正如我们所知道的，it引导一个形式主语从句，，所以要这么选