$ apt-cache policy plasma-desktop
plasma-desktop:
  Installed: (none)
  Candidate: 4:5.6.5-0ubuntu1~ubuntu16.04~ppa1
  Version table:
     4:5.6.5-0ubuntu1~ubuntu16.04~ppa1 500
        500 http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu xenial/main amd64 Packages
     4:5.5.5-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages