asp.net 数据绑定问题

利用AUTOPOSTPACK 来选中省，第二个droplist 跳到对应省的市

我做出来后选择第二个省时，跳不出该省的市，还是停留在第一个省
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection("server=.;uid=sa;pwd=4184085dsc;database=sheng;");
        con.Open();
        SqlCommand cmd=new SqlCommand("select * from one",con);
        SqlDataReader sdr = cmd.ExecuteReader();
        DropDownList1.DataSource = sdr;
        DropDownList1.DataTextField = "pro";
        DropDownList1.DataValueField = "id";
        DropDownList1.DataBind();
        sdr.Close();
     
        SqlCommand cmd1 = new SqlCommand("select * from two", con);
        SqlDataReader sdr1 = cmd1.ExecuteReader();
        DropDownList2.DataSource = sdr1;
        DropDownList2.DataTextField = "cityname";
        DropDownList2.DataValueField = "cityid";
        DropDownList2.