"ctrl+space" has been bound to invoke some input method and does not work in Emacs

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http://askubuntu.com – In Emacs run in terminal, ctrl+space is a standard shortcut key to select region. But under Ubuntu 12.04, it will invoke some input method for non-English language. I was wondering if I can reserve it for Emacs only? Under System Settings -> Keyboard -> Shortcuts, I couldn't find to what ctrl+space has been bound. Strange. Thanks! (HowTos)