how to bind dropdown value to datagrid?

 <form id="form1" runat="server">
    <div>
    <asp:ScriptManager ID="sm" runat="server"></asp:ScriptManager>
     <asp:UpdatePanel runat="server">
        <ContentTemplate>
            <table>
                <tr>
                    <td>
                        <asp:DropDownList ID="ddl1" runat="server" AutoPostBack="false">
                            <asp:ListItem Text="Sector1" Value="Sector1"></asp:ListItem>
                            <asp:ListItem Text="Sector2" Value="Sector2"></asp:ListItem>
                        </asp:DropDownList>
                    </td>
                    <td>
                        <asp:DropDownList ID="ddl2" runat="server" AutoPostBack="false">
                            <asp:ListItem Text="Singapore" Value="Singapore"></asp:ListItem>
                            <asp:ListItem Text="General" Value="General"></asp:ListItem>
                        </asp:DropDownList>
                    </td>
                    <td><asp:Button ID="btnAdd" Text="Add" runat="server" onclick="btnAdd_Click" /></td>
                </tr>
                <tr>
                    <td colspan="3">
                        <asp:GridView ID="grdDetails" runat="server" AutoGenerateColumns="false" EnableViewState="true">
                            <Columns>
                                <asp:BoundField DataField="Sector" HeaderText="Sector" />
                                <asp:BoundField DataField="NewSector" HeaderText="News Sector Type" />
                            </Columns>
                        </asp:GridView>
                    </td>
                </tr>
            </table>
        </ContentTemplate>
     </asp:UpdatePanel>
    </div>
    </form>

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 public partial class _Default : System.Web.UI.Page
    {
        DataTable dtSectors;
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btnAdd_Click(object sender, EventArgs e)
        {
            CreateDataSource();
            if (grdDetails.Rows.Count > 0)
            {
                LoadDataBackFromGrid();
            }
            
            AddNewRow();

            grdDetails.DataSource = null;
            grdDetails.DataSource = dtSectors;
            grdDetails.DataBind();
        }

        private void AddNewRow()
        {
            DataRow dr = dtSectors.NewRow();
            dr["Sector"] = ddl1.SelectedItem.Text;
            dr["NewSector"] = ddl2.SelectedItem.Text;
            dtSectors.Rows.Add(dr);
        }

        private void LoadDataBackFromGrid()
        {
            DataRow dr;
            foreach (GridViewRow row in grdDetails.Rows)
            {
                dr = dtSectors.NewRow();
                for (int i = 0; i < dtSectors.Columns.Count; i++)
                {
                    dr[i] = row.Cells[i].Text;
                }
                dtSectors.Rows.Add(dr);
            } 

        }

        private void CreateDataSource()
        {
            dtSectors = new DataTable();
            dtSectors.Columns.Add("Sector", typeof(System.String));
            dtSectors.Columns.Add("NewSector", typeof(System.String));
        }
    }

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