Add spaces: Donotchangeanylinesthatbeginwithdebcdrom:.Doingsowouldinvalidatethelineandyouwouldhavetorunapt-cdromagain.Donotbealarmedifa'cdrom'sourcelinerefersto"unstable".Althoughconfusing,thisisnormal.
Do not change any lines that begin with deb cdrom:. Doing so would invalidate the line and you would have to run apt-cdrom again. Do not be alarmed if a 'cdrom' source line refers to " unstable " . Although confusing, this is normal.