C
此题考查名词性从句。that,it和which都不能在句中作成分引导主语从句，排除A、B、D三项。题意为“他们在森林里迷路了，更糟糕的是天开始黑了”。汉语中的“更糟糕的” 可用worse still,to make matters worse,what made matters worse等表达。但worse still和to make matters worse一般用作状语，而what made matters worse常作主语从句。此题中明显是作主语，故答案为C项。此句可改为:They lost their way in the forest.Worse still,night began to fall.或They lost their way in the forest,and to make matters worse,night began to fall.