problem during reinstalltion of ubuntu software center

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http://askubuntu.com – i had some problem with ubuntu software center so i uninstalled it then reinstalled using following codes sudo apt-get install software-center i was getting the message as follows....... Package software-center is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'software-center' has no installation candidate can some one please help with this problem???? (HowTos)