题目内容：

A man cannot really be happy if _he enjoys doing is ignored by society as of no value .
A.what B.when C.that D.all what 为什么,求详解

优质解答

考查主语从句
这是一个主从复合句
主句是A man cannot really be happy
从句是由if引导的条件状语从句,在从句if _he enjoys doing is ignored by society as of no value中,is前面的是主语,因此这里我们要选择一个词来引导主语从句,doing是一个及物动词,后面缺宾语,因此要填连接代词,指物通常用what.
如果理解为定语从句,if all （that） he enjoys doing is ignored by.这里all是先行词,关系代词that作宾语指物,也可以省略.
因此这题的答案 what= all that=all