定于从句This is the best book _____ I have been looking for all this year.A.who B.whomC.which D./选D为什么不选C?

定于从句
This is the best book _____ I have been looking for all this year.
A.who B.whomC.which D./
选D为什么不选C?

有the best 最高极,只能写THAT或什么都不写