Dropdown Value is getting reset in Gridview Item Template

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ASP.NET

DropDownList

Hi..
I am using dropdownlist in the gridview control where the value is getting reset.
Following is my code:-

<asp:GridView ID="gdvRates" runat="server" AutoGenerateColumns="False" CssClass="style1"
        OnRowDataBound="gdvRates_RowDataBound">
        <Columns>
            <asp:TemplateField>
                <HeaderTemplate>
                    Commodity</HeaderTemplate>
                <ItemTemplate>
                    <asp:DropDownList ID="ddlCommodity" runat="server" class="style1"
                     >
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField>
                <HeaderTemplate>
                    City</HeaderTemplate>
                <ItemTemplate>
                    <asp:DropDownList ID="ddlCity" runat="server" >
                    </asp:DropDownList>
                   
                </ItemTemplate>
            </asp:TemplateField>
 </Columns>
    </asp:GridView>

And the following is server code where am poplulating the dropdowns:-

  protected void gdvRates_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                DropDownList ddlCommodity = (e.Row.FindControl("ddlCommodity") as DropDownList);
                ddlCommodity.Items.Clear();
                objDt = objBLL.GetSubCommodities();
                ddlCommodity.Items.Add("Commodity");
                foreach (DataRow dr in objDt.Rows)
                {
                    ddlCommodity.Items.Add(new ListItem(dr["Subcommodity_name"].ToString(), dr["Subcommodity_id"].ToString()));
                }

                DropDownList ddlCity = (e.Row.FindControl("ddlCity") as DropDownList);
                ddlCity.Items.Clear();
                objDt = objBLL.GetCityNames();
                ddlCity.Items.Add("City");
                foreach (DataRow dr in objDt.Rows)
                {
                    ddlCity.Items.Add(new ListItem(dr["City_name"].ToString(), dr["city_id"].ToString()));
                }
}
}

Please let me know what am i missing?? Thanks.