1)What's tha hig building?
____ we borrow books or do reading.
A:That's where B:The building where
2)It's not unusual ___ a generation gap between teachers and students.
A:for there to be B:there to be
　想要理由．

1)What's tha hig building?（那高楼是做什么用的？）
____ we borrow books or do reading.（那是我们借书或阅览书籍的地方）
A:That's where B:The building where
答案是A。 That's where we borrow books or do reading。
That是主语，is 是谓语， where we borrow books or do reading 是表语，是一个由where 引导的表语从句。
如果选B:The building where， 则答非所问。

2)It's not unusual ___ a generation gap between teachers and students.
A:for there to be B:there to be
答案是A。
可以看做一个句型。It's + adj. + for + n.\pron. + to do sth.
eg. It's difficult for me to learn French well.
It's easy for the boy to carry the box.