How Can I remove   from my Excel file

Response.Clear();
        Response.Buffer = true;
        Response.AddHeader("content-disposition",
         "attachment;filename=GridViewExport.csv");
        Response.Charset = "";
        Response.ContentType = "application/text";

        //GridSearchdetails.AllowPaging = false;
        //GridSearchdetails.DataBind();



        //now we want to write the columns headers of the table
        StringBuilder sb = new StringBuilder();
        for (int k = 0; k < GridView1.Columns.Count; k++)
        {
            //add separator
            sb.Append(GridView1.Columns[k].HeaderText + ',');
        }
        //append new line
        sb.Append("\r\n");
        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
            for (int k = 0; k < GridView1.Columns.Count; k++)
            {
                //add separator
                //sb.Append(GridView1.Rows[i].Cells[k].Text + ',');
                string abc = GridView1.Rows[i].Cells[k].Text;
                abc.Trim();
                if (abc == "&nbsp;")
                {
                    abc = "";
                }
                if (abc == "")
                {

                }
                sb.Append(abc + ',');
            }
            //append new line
            sb.Append("\r\n");
        }
        Response.Output.Write(sb.ToString());
        Response.Flush();
        Response.End();