There was nothing to do（ ） wait until he came back.
A.but
B.but to
可是but前面是to do,根据平行结构,后面也应该是but to wait 为什么选A呢?
当but/except解释为“除了”时,如果后面跟上动词,有两种情况,一种是用to +动词原形,另一种就是直接跟上动词原形.判定原则是这样的：
当but/except前有动词do这个词的任何一种形式的时候,在but/except后不加to,直接跟上动词原形；
当but/except前没有do这个词的时候,那么后面就要用to+动词原形.
如：He could do nothing but wait.
He had nothing to do but wait.
He had no choice but to wait.
另外,还有几个常用的短语也要记住：
can't but do; can't hlep but do 这两个短语都解释为“不得不做……
can't help doing 情不自禁做……；忍不住做……
can't hlep to do 不能帮助做某事
如：She can't help to clean the room because she's busy making a cake.
While shopping,people sometimes can't help being persuaded into buying something they don't really need.
