关于which和what的表语从句语法

问题描述:

关于which和what的表语从句语法
the book is which on the desk 和the book is what on the desk哪个对?
This problem is which the teacher explained yesterday 这句话为什么不能算是表语从句?
如果算的话 Is this problem ( )the teacher explained yesterday?
A that B the one C which 那为什么不选which

选B,上面那个都有点读不通啊