there are five pairs to choose from ,but i am at a loss which to buy
问题描述：
there are five pairs to choose from ,but i am at a loss which to buy
there are five pairs______,but i'm at a loss which to buy.
choose from
为什么不能选for choosing
答
to do sth 是不定式~表能力（能够） 目的~
有五双可以选择,我却不知道选哪双.
for doing sth 表“为了.”