( )troubles us most is ( )we do not know what todo for him.

A it......the thing
B what......that
C that......it
D the thing ......that
答案是：B （为什麽不是 It......that)

句子中有troubles、is、do not know三个动词，则其中两个必为从句成分。其句式为“…is…”。
“（）troubles us most”差主语，此为名词性从句，用what来引导。“( )we do not know what to do for him”为句中宾语，为名词性从句，但又从句的成分齐全，用that来引导。

如果句子写成 (it) troubles us most is (that) we do not know what to do for him. that来引导宾语从句，则句子的结构就成了it troubles us most is sth，那么句子中就出现了两个动词troubles和is，而在英语的一个句子中是步能出现两个动词的。