the tower of London is a famous historic building. A lecture is to be given about it

`1`. The tower of London is a famous historic building. A lecture is to be given about ii.

`->` The tower of London, which a lecture is to be given about, is a famous historic building.

`-` "which" dùng thay thế cho vật trong mệnh đề quan hệ 

`-` "The tower of London" là danh từ riêng nên dùng mệnh đề quan hệ không xác định