how to set the PrintOptions?

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how can i set the Paper orientation

 Microsoft.Office.Interop.PowerPoint.Presentation work = null;             
            Microsoft.Office.Interop.PowerPoint.Application app = new Microsoft.Office.Interop.PowerPoint.Application(); 
  work = presprint.Open(filename, Microsoft.Office.Core.MsoTriState.msoCTrue, Microsoft.Office.Core.MsoTriState.msoCTrue, Microsoft.Office.Core.MsoTriState.msoFalse); 
            work.PrintOptions.PrintInBackground = 0;

there is no Paper orientation set in the printoptions... ...