问题补充：
is known to us all is that the old worker, for life was hard in the past, still works hard in his seventies.
A. As; whom
B. What; whose
C. It; whose
D. What; whom
答案：
A. As; whom
B. What; whose
C. It; whose
(答案→)D. What; whom
解析：考查名词性从句和非限制性定语从句。As引导非限制性定语从句放句首时，后常有逗号与主句分开，is known to us all后无逗号，排除A；it作形式主语的句型是it is known to us all that...，句中that前有is，排除C；主语从句缺少主语，用what，构成what ... is that...句型。非限制性定语从句中的life是主语，即life was hard for the old worker。用whose的话，for whose life是介词短语，从句缺少主语。故选D。
