If burndown charts are used to visualize progress, what does a trend line through a release burndown chart indicate?

The evolution of the cost spent on the project.

When all work will be completed to the Scrum Team can be released for other work.

When the work remaining will likely be completed if nothing changes on the Product Backlog or the Development Team.

When the project will be over if the Product Owner removes work that is equal in effort to any new work that is added.