How to load Dropdown list in selected event bind the Data from Db.

hi
I have created simple Sample for Dropdownlist load and selected event another dropdownlist loaded data from the fetch db and bind them using
Update Panel

Client Side Coee

  <form id="form1" runat="server">
    <div>
    <asp:UpdatePanel ID="up1" runat="server" >
    <ContentTemplate>
    
    
    From 
    <asp:DropDownList ID="Drp1" runat="server" AutoPostBack="True" 
            onselectedindexchanged="Drp1_SelectedIndexChanged">
    </asp:DropDownList>

    To
    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" onselectedindexchanged="DropDownList1_SelectedIndexChanged" 
            >
    </asp:DropDownList>

    <asp:LinkButton ID="lnkClear" runat="server" Text ="Clear" onclick="lnkClear_Click"></asp:LinkButton>
    </div>

    </ContentTemplate>
    </asp:UpdatePanel>
    </form>

Server Side c# Code

 SqlConnection sqlcon = new SqlConnection(ConfigurationManager.ConnectionStrings["DbConnect"].ConnectionString);
    protected void Page_Load(object sender, EventArgs e)
    {

        if (!IsPostBack)
        {

            DataTable dtt = new DataTable();
            sqlcon.Open();
            SqlDataAdapter sqladp = new SqlDataAdapter(" Select * from tbl_Time", sqlcon);
            sqladp.Fill(dtt);
            DateTime ddd = new DateTime();

            Drp1.DataSource = dtt;
            Drp1.DataTextField = "Name";
            Drp1.DataValueField = "Id";
            Drp1.DataBind();
            DropDownList1.Items.Insert(0, "Select");
        }
    }
    protected void Drp1_SelectedIndexChanged(object sender, EventArgs e)
    {

        DataTable dtt = new DataTable();
        //sqlcon.Open();
        SqlDataAdapter sqladp = new SqlDataAdapter(" Select * from tbl_Time where id="+ Convert.ToInt32(Drp1.SelectedValue)  +"", sqlcon);
        sqladp.Fill(dtt);

        DropDownList1.DataSource = dtt;
        DropDownList1.DataTextField = "Name";
        DropDownList1.DataValueField = "Id";
        DropDownList1.DataBind();
        DropDownList1.Items.Insert(0, "Select");
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        DataTable dtt = new DataTable();
       //sqlcon.Open();
        SqlDataAdapter sqladp = new SqlDataAdapter(" Select * from tbl_Time where id=" + Convert.ToInt32(Drp1.SelectedValue) + "", sqlcon);
        sqladp.Fill(dtt);

        Drp1.DataSource = dtt;
        Drp1.DataTextField = "Name";
        Drp1.DataValueField = "Id";
        Drp1.DataBind();
        DropDownList1.Items.Insert(0, "Select");
    }
    protected void lnkClear_Click(object sender, EventArgs e)
    {
        DataTable dtt = new DataTable();
        //sqlcon.Open();
        SqlDataAdapter sqladp = new SqlDataAdapter(" Select * from tbl_Time", sqlcon);
        sqladp.Fill(dtt);
        DateTime ddd = new DateTime();

        Drp1.DataSource = dtt;
        Drp1.DataTextField = "Name";
        Drp1.DataValueField = "Id";
        Drp1.DataBind();
        DropDownList1.Items.Insert(0, "Select");
    }