Enable Disable TextBox inside GridView based on DropDownList Selection in ASP.Net

Please refer this

<form id="form1" runat="server">
<div>
    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
        <asp:ListItem Text="Please Select" Value="0" />
        <asp:ListItem Text="India" />
    </asp:DropDownList>
    <asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
        runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="Id" HeaderText="Id" ItemStyle-Width="30" />
            <asp:TemplateField HeaderText="Enter Degree Name">
                <ItemTemplate>
                    <asp:TextBox ID="txtCity" runat="server" Text='<%# Eval("CityName") %>' />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
</form>

C#:

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2] { new DataColumn("Id", typeof(int)),
                    new DataColumn("CityName", typeof(string))});
        dt.Rows.Add(1, "Mumbai");
        dt.Rows.Add(2, "Delhi");
        dt.Rows.Add(3, "Kolkata");
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (this.DropDownList1.SelectedItem.Value == "0")
    {
        foreach (GridViewRow row in this.GridView1.Rows)
        {
            TextBox tb = row.FindControl("txtCity") as TextBox;
            tb.Text = string.Empty;
            tb.Enabled = false;
        }
    }
}

Thank You.