题目内容：

There are five pairs___,but I'm at a loss which to buy.
A.to be chosen B.to choose from C.to choose D.for choosing.请问各位大虾为何选答案B而不是A呀,

优质解答

先看后一句,but I'm ……说明“我”是主语.
又因为A选项是被动语态而B选项是主动语态且from介词在choose做后置定语.
最后在看which to buy显然此人不知该选哪一双,用choose from更可以体现出此人想要从中选一双.
另外,也可以根据语感来判断!