i could not add a bookmark to the right of a text

InsertTextAtSelection("Hellow 0\n");
MoveDownNLine(1);
InsertTextAtSelection("Hellow 1\n");
MoveDownNLine(2);
InsertTextAtSelection("Hellow 2\n");

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using Word = Microsoft.Office.Interop.Word;
//http://msdn.microsoft.com/en-us/library/2a9dt54a.aspx

namespace EstemaraA4Shmrani
{
    public partial class Form1 : Form
    {
        Word.Application wrdApp;
        Word._Document wrdDoc;
        Word.Range rng;
        //
        Object oMissing = System.Reflection.Missing.Value;
        Word.Bookmark bookmarkA;
        Object name;
        //
        Object unit, count, extend;
        Object start, end;
        Word.Selection wrdSelection;
        //-------------------------
        public Form1()
        {
            InitializeComponent();
        }
        //---------------------Word-----------------
        private void CallWord_Click(object sender, EventArgs e)
        {   
            wrdApp = new Word.Application();
            wrdApp.Visible = true;
            Object range = rng;

            name = "D:\\EstemaraA4Sabtan.docx";
            wrdDoc = wrdApp.Documents.Open(ref name, ref oMissing,
                    ref oMissing, ref oMissing, ref oMissing, ref oMissing,
                    ref oMissing, ref oMissing, ref oMissing, ref oMissing,
                    ref oMissing, ref oMissing, ref oMissing, ref oMissing,
                    ref oMissing, ref oMissing);
            //----------------------
            InsertTextAtSelection("Hellow 0\n");
            InsertTextAtSelection("Hellow 1\n");
            InsertTextAtSelection("Hellow 2\n");          
            // Display Bookmarks.
            wrdDoc.ActiveWindow.View.ShowBookmarks = true;
            MoveUpNLine(2); InsertTextAtSelection("Hellow A bookmarkA\n");
            MoveNcharacterLeft(9);
            //SelectN_Words(1);
            //Supply a Start and end value for the Range.
            start = 9 * 2 ;
            end =   9 * 3 ;
            rng = wrdDoc.Range(ref start, ref end);
            rng.Select();
            //----------
            range = rng;
            bookmarkA = wrdDoc.Bookmarks.Add("This_is_the_name_of_bookmark_in_ms_word", ref range);
            rng.InsertAfter(" after ");
            //
            //Release References.
            wrdSelection = null;
            wrdDoc = null;
            wrdApp = null;
        }//
        //------------------------
        private void MoveUpNLine(Object id)
        {
            unit = Word.WdUnits.wdLine;
            extend = Word.WdMovementType.wdMove;
            wrdApp.Selection.MoveUp(ref unit, ref id, ref extend);
        }
        //------------------------
        private void MoveDownNLine(Object id)
        {
            unit = Word.WdUnits.wdLine;
            extend = Word.WdMovementType.wdMove;
            wrdApp.Selection.MoveDown(ref unit, ref id, ref extend);
        }
        //--------------------------------------------------
        private void MoveNcharacterLeft(Object n)
        {// Move the insertion point left n characters.
            unit = Word.WdUnits.wdCharacter;
            count = n;
            extend = Word.WdMovementType.wdMove;
            wrdApp.Selection.MoveLeft(ref unit, ref count,
                ref extend);
        }
        //-------------
        private void MoveNcharacterRight(Object n)
        {// Move the insertion point left n characters.
            unit = Word.WdUnits.wdCharacter;
            count = n;
            extend = Word.WdMovementType.wdMove;
            wrdApp.Selection.MoveRight(ref unit, ref count,
                ref extend);
        }
        //-------------
        private void InsertTextAtSelection(string Mystring)
        {
            Word.Selection sln = wrdApp.Selection;
            // Make sure overtype is turned off.
            wrdApp.Options.Overtype = false;
            //if (sln.Type == Word.WdSelectionType.wdSelectionNormal)
            //{
            sln.TypeText(Mystring);
            //sln.TypeParagraph();
            //}
        }
        private void SelectSentencesId(int id)
        {
            if (wrdDoc.Sentences.Count >= id)
            { //Supply a Start and end value for the Range.
                start = wrdDoc.Sentences[id+1].Start;
                end = wrdDoc.Sentences[id+1].End;
                rng = wrdDoc.Range(ref start, ref end);
                rng.Select();
            }
        }
        private void SelectN_Words(Object n)
        {// Select the 3 words to the right of the insertion point.
        unit = Word.WdUnits.wdWord;
        count = n;
        extend = Word.WdMovementType.wdExtend;
        wrdApp.Selection.MoveRight(ref unit,ref count,ref extend);
        }
    }
}