qw@ubuntu:~ $ sudo apt-get install virtualbox
password for qw [sudo]:
Reading lists of packages... It is ready
Creation of a tree of dependences
Reading information on a condition... It is ready
The virtualbox package is inaccessible, but will mention in the list of dependences of other package.
It can mean that the package is absent, obsolete, or is available from the sources which have not been mentioned in sources.list
E: For a virtualbox package candidates for installation aren't found
qw@ubuntu:~ $