There is a small garden ___ the teaching building of our uni

There is a small garden ___ the teaching building of our university
A in the back of B after C above Dat the back of
the.跟 at the 有什么区别么.
There is a firm projector___the hall of our university
A in the back of B at the back of C after Dbeside
比较一下两个题的区别,