[O] how to hide titles marked with "DONE"

[bofe]

Hi,
        I use org to manage my projects. When some works are finished ,I marked 
the titles “DONE”,but as time goes by ,there are too many of them, that I can’t 
focus on the rest of unfinished works.
        Is there a way to hide the “DONE" titles,rather than delete them?

        Thanks a lot!
        -
        bofe