การใช้ Sessionในเอาออปเจ๊ค RadioButtonList การเขียนโค้ดยังไงครับขาดอยู่ตัวเดียว

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

public partial class _Default : System.Web.UI.Page
{
    private SqlConnection CN()
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = ConfigurationManager.ConnectionStrings["AAA"].ToString();
        return con;
    }
    DataSet ds;
    string sql;
    protected void Page_Load(object sender, EventArgs e)
    {
            if (!this.IsPostBack)
            {
                BindData();
            }
    }
    protected void BindData()
    {
            sql = "select*from student";
            SqlCommand cm = new SqlCommand(sql, CN());
            ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(sql, CN());
            da.Fill(ds, "student");
            DropDownList1.DataSource = ds.Tables["student"];
            DropDownList1.DataTextField = "sname";
            DropDownList1.DataValueField = "sname"; // ต้อวเป็น PK radio checkbox dropdownlist listbox
            DropDownList1.DataBind();

            RadioButtonList1.DataSource = ds.Tables["student"];
            RadioButtonList1.DataTextField = "stel";
            RadioButtonList1.DataValueField = "sname";
            RadioButtonList1.DataBind();
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        //Response.Redirect("default2.aspx?");
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        
    }
    protected void Button1_Click1(object sender, EventArgs e)
    {
	Session["A"] = TextBox1.Text;
	Session["B"] = TextBox2.Text;
	Session["C"] = DropDownList1.SelectedItem.ToString();
	Session["D"] = RadioButtonList1.SelectedItem.ToString();

	Response.Redirect("default2.aspx?");
    }
}