What we all know is that the old gentleman for

问题补充：

What we all know is that the old gentleman, for

答案：

D whom非限制性定语从句的一种.翻译为：(对他来说)过去的生活十分艰苦.for whom属于介宾结构充当状语,life was hard in the past句子完整.

======以下答案可供参考======

供参考答案1:

A)whose life就是指的是那个old gentleman的Life,两个逗号间是插入语。