Can't install g++ in Software Center

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http://askubuntu.com – 1) I can't install g++ in software center. 2) I can install other programs. For example, after I found g++ can't be installed, I installed chrome for test purpose through software center, and it worked! 3) g++ can be installed in the terminal via apt-get install. Only the software center way didn't work. Who can tell me why this happens? How can I fix it? PS: I have had gcc and gdb installed in my 12.04LTS before. My goal is to install Qt Creator via software center, which doesn't work either. (HowTos)