求教：我在Lubuntu12.04上不能安装ubuntu软件中心，请热心人帮忙！

[qq510707415]

如题，我用命令 sudo apt-get install software-center,出现以下提示：

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更换介质：请把标有
“Lubuntu 12.04 _Precise Pangolin_ - Release i386 (20120423.1)”
的盘片插入驱动器“/media/cdrom/”再按回车键
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我的系统是Lubuntu12.04
系统安装在U盘上
在安装其它软件的时候一切正常，请高手指点，为什么不能装软件中心。

[mraandtux]

去软件源设置修改源

[qq510707415]

去软件源设置修改源

应该修改为什么啊？