dont want to pass dropdown list value and the paramter in the query string if NULL ???

I am passing  selected values of dropdownlists from search.aspx to results.aspx
   {           Response.Redirect("Results.aspx?CountryId=" + D3.SelectedItem.Value.ToString()             + "&AssignmentId=" + D4.SelectedItem.Value.ToString() + "&EthnicityId=" + D2.SelectedItem.Value.ToString());           }

On Results.aspx is method that accepts parameters and generates dynamic Select statement.

If user does not enter a value in DropDown d3 , i dont want to pass the CountryId=" + D3.SelectedItem.Value.ToString()           in the response redirect as I get an error on results.aspx.  

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