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?