Question

I don't know ( )I should choose.A,why B,when C,which D,what,求详解

Answer 1

选D。如果要选A或B，choose后面应该加上宾语，
如：I don't know ( why/when)I should choose it.
剩下的C可能是比较有迷惑性的，因为我们都比较习惯说 choose which one，
但是要注意到 which 后面也要加名词，如：I don't know (which pen )I should choose,
而what就是等同于which+名词。

追问

只有what 是关系代词，其他都是关系副词对吗

追答

what是关系代词，因为它可以代替which one，which，when,why都是关系副词。