There are three pens in the box?

There are three pens in the box?
There are three pens in the box,which one is yours,this one or that one or the one in the pencil box
请问:主语——there
系动词——are
表语——three pens
地点状语——in the box
请问：this one or that one or the one in the pencil box 在整个的句子中的成分是什么,是对整个句子进行“补充说明”的吗?