Question

There are so many interesting toys that I can't decide ___.A,to buy which.B.

There are so many interesting toys that I can't decide ___.
A,to buy which.B.which to buy C.to buy what D. what to buy.
选B，为什么不能选D

Answer 1

B.which to buy 是对的。
题目隐含着选择的范围，故要用which，不能用what。

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追问

可我觉得which one to buy 才对啊
那D什么时候可以用

追答

没错，which one to buy 也对.which=which one 。
没有选择的范围，就用what。

Answer 2

因为有前边的语境There are so many interesting toys
所以用which to buy,去买哪个而不是去买什么。