Identify selected Master slides from powerpoint

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C#2.0

C#

AddIn

Hi,
I have created an application with C# .net and powerpoint2007 (vsto addin), i am using code

Presntation.SlideMaster.Background.Fill.UserPicture(PictureFileName); 

to set background of SlideMaster, but its apply only the first SlideMaster in a presentation. when i create 2 or more SlideMaster in a single presentation , i need to select the current SlideMaster and apply the picture as background.How can i locate selected SlideMaster from a presenation and set background using c# code..Also i need to set the background of a perticular slide under master slide ....
Pls help me.