Identify the subordinate clause. The book i am reading concerns world population problems

Im not completely sure but i think the subordinate clause is "concerns the world population"

A subordinate clause is a dependent clause that cannot stand on its own.

The subordinate clause here is I am reading.

The whole sentence is The book (that) I am reading concerns world population problems.

As you can see, the independent sentence is The book concerns world population problems, because it can stand on its own.

That I am reading cannot.