英语句子成分划分：下面句子哪个是主语补足语?
问题描述：
英语句子成分划分：下面句子哪个是主语补足语?
What appear to be large windows in the second storey are glass heat collector.
哪是主句?主语补足语是哪个?麻烦帮我把句子成分划分一下啦,
答
What are glass heat collector 是主句
in the second storey是状语
What appear to be large windows是主语从句
appear是从句里的谓语
to be large windows和appear 组成系表结构为什么to be large windows和appear 组成系表结构？？