How to copy a part of word document using interop

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.NET

I need to identity a part of word document with a key then in need to copy it into another word document. how can i achieve it through C#.net.

i tried the following it result as text i need it as document
and paragraph should be i identified with a key word

What I have tried:

public string ReadFileContent(string path, int paraGraphNum)
        {
            int i = 0;

            StringBuilder sb = new StringBuilder();

            Microsoft.Office.Interop.Word.Application wordApp = new Microsoft.Office.Interop.Word.Application();

            object file = path;

            object nullobj = System.Reflection.Missing.Value;

            Microsoft.Office.Interop.Word.Document doc = wordApp.Documents.Open
                                                    (ref file, ref nullobj, ref nullobj,
                                                    ref nullobj, ref nullobj, ref nullobj,
                                                    ref nullobj, ref nullobj, ref nullobj,
                                                    ref nullobj, ref nullobj, ref nullobj);


            Microsoft.Office.Interop.Word.Paragraphs DocPar = doc.Paragraphs;

            // Count number of paragraphs in the file
            long parCount = DocPar.Count;
           
            // Step through the paragraphs
            while (i < parCount)
            {
                i++;
                if (i == paraGraphNum)
                {
                    sb.Append(DocPar[i].Range.Text);
                    break;
                }
            }

            doc.Close(ref nullobj, ref nullobj, ref nullobj);
            wordApp.Quit(ref nullobj, ref nullobj, ref nullobj);
            return (sb.ToString());
        }
    }