however,for ,but ,and though,although,so词语的用法及区别

however,for ,but ,and though,although,so词语的用法及区别
每一个词都举些例子,我现在还混在里面.有好的加分!
顺便问一下：-Can you help me with my physics lesson this afternoon,? -I'm soory,______ I've something important to do.
A.yet(肯定不可能吧) B.but(我感觉不大想) C.however D.for (这两个就不清楚了)