Which concrete package gets installed by default for a virtual package?

[Ryan Nowakowski <tubaman@fattuba.com>]

My google-fu is failing me here.  I have a set of packages like this:

    Package: foo
    Architecture: all
    Depends: bar
    Description: my meta package
    
    Package: blerg
    Architecture: any
    Provides: bar
    Conflicts: bar
    Replaces: bar
    Description: One concrete package that provides bar
    
    Package: blerg-plus
    Architecture: any
    Provides: bar
    Conflicts: bar
    Replaces: bar
    Description: Another concrete package that provides bar

If I install foo, which concrete package providing bar gets installed,
blerg or blerg-plus?  How can I control which concrete package(blerg or
blerg-plus) gets installed when I install foo?

Thanks!

Ryan N