求助，急！用PlaceHolder存放多次生成的RadioButtonList，只有第一次选中选项并点击按钮后能读取到rbl的SelectedValue值，第二次开始就读不到了，一直显示为""

[Fish_Zhou1]

红色部分的SelectedValue在运行第二次之后就不能读取，求高手解答！

string a = "";
        protected void Page_Load(object sender, EventArgs e)
        {

            string sql = "select * from item where pollid=2;";

            SqlConnection con = new SqlConnection();
            con.ConnectionString = "server=(local);database=vote;uid=sa;pwd=123456";
            con.Open();
            SqlCommand com = new SqlCommand();
            com = new SqlCommand();
            com.Connection = con;
            com.CommandText = sql;

            SqlDataReader reader = com.ExecuteReader();

            RadioButtonList rbl = new RadioButtonList();
            rbl.DataSource = reader;
            rbl.DataTextField = "Title";
            rbl.DataValueField = "itemid";
            rbl.DataBind();
            plbody.Controls.Add(rbl);
           
            con.Close();
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            Control body = plbody.Controls[0];
            RadioButtonList rbl = (RadioButtonList)body;
            a = rbl.SelectedValue;
            plbody.Controls.Clear();

            string sql = "select * from item where pollid=3;";

            SqlConnection con = new SqlConnection();
            con.ConnectionString = "server=(local);database=vote;uid=sa;pwd=123456";
            con.Open();
            SqlCommand com = new SqlCommand();
            com = new SqlCommand();
            com.Connection = con;
            com.CommandText = sql;

            SqlDataReader reader = com.ExecuteReader();

            rbl.DataSource = reader;
            rbl.DataTextField = "Title";
            rbl.DataValueField = "itemid";
            rbl.DataBind();
            plbody.Controls.Add(rbl);

            con.Close();