they lost their way in the forest and___made matters worse was that night began to fall.

they lost their way in the forest and___made matters worse was that night began to fall.
A) that B) it C) what D) which
为什么选择C呢.I do not know why.
that 只起连接作用，在句中不充当成分。what 为连接代词，引导主语从句，且充当成分。
it 可以做形式主语，which又有什么限制呢？为什么不选it 和wihch呢？
and前后连两个完整句子，所以单独拿后一句分析：
___made matters worse was that night began to fall.
其中was是谓语，that从句为表语从句，所以前面的___made matters worse是主语从句，而这个从句里缺了主语，所以用what.
which强调两者选一个。而it不能引导从句，可以做形式主语。