the pen that you want is on the desk 能不能用which 为什么

题目

the pen that you want is on the desk 能不能用which 为什么
为什么为什么

答案

可以用which.
先行词（the pen）指物时,关系代词用that 或which.在此句中,关系代词在从句中充当宾语,也可省略.
即：the pen that/which/省略 you want is on the desk.