Problem with installing gnome

[Lian Liming]

Hi all,
     I am using debian/unstable, and find i can't install gnome with 
the following error:

#####aptitude install gnome
Reading Package Lists... Done
Building Dependency Tree
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
E: Unable to correct problems, you have held broken packages.
E: Unable to correct dependencies, some packages cannot be installed
E: Unable to resolve some dependencies!
Some packages had unmet dependencies.  This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following packages have unmet dependencies:
 gnome: Depends: gnome-desktop-environment (= 61) but it is not installable
        Depends: gnome-office (= 61) but it is not installable
        Depends: evolution but it is not installable or
                 balsa but it is not installable
        Depends: gnome-cups-manager but it is not installable
        Depends: gnome-system-tools (>= 0.90.0) but it is not installable
        Depends: magicdev but it is not installable or
                 gnome-volume-manager but it is not installable
        Depends: rhythmbox but it is not installable
        Depends: totem but it is not installable

There are some errors with dependence, how can i fix it?
Thank you for suggestions.
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