We should do as much as we can ___ water. A.for saving B.to save C.save D.tsaved

We should do as much as we can ___ water. A.for saving B.to save C.save D.tsaved
We should do as much as we can ___ water.
A.for saving B.to save C.save D.tsaved
答案给的是B,但我想知道C为什么不对,can不是情态动词吗,既然是后面为什么还要先加to再加动词.
还有就是The man ran out ___（see） what was wrong.为什么要填to see？这里有什么语法讲究吗？谢谢。