i had a problem in drop down list

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 addexams : System.Web.UI.Page
{
    #region
    string c = ConfigurationManager.ConnectionStrings["eexam"].ConnectionString;
    int i;
    #endregion

    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection sc = new SqlConnection(c);
        sc.Open();
        SqlCommand scom = new SqlCommand("select category from addexams", sc);
        SqlDataReader d = scom.ExecuteReader();
        if (d.HasRows)
        {
            while (d.Read())
            {
                RemoveDuplicateItems(DropDownList1);
                DropDownList1.Items.Add(d.GetString(0));

            }
        }

        sc.Close();
    }

      
    protected void Button1_Click(object sender, EventArgs e)
    {
        Response.Redirect("add.aspx");
    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection sc = new SqlConnection(c);
        sc.Open();
        SqlCommand scom = new SqlCommand("select category from addexams", sc);
        SqlDataReader d = scom.ExecuteReader();
        if (d.HasRows)
        {
            while (d.Read())
            {
               RemoveDuplicateItems(DropDownList2);
                DropDownList2.Items.Add(d.GetString(0));

            }
        }

    //    sc.Close();
    //}

    public static void RemoveDuplicateItems(DropDownList ddl)
    {
        for (int i = 0; i < ddl.Items.Count; i++)
        {
            ddl.SelectedIndex = i;
            string currValue = ddl.SelectedItem.ToString();
            for (int counter = i + 1; counter < ddl.Items.Count; counter++)
            {
                ddl.SelectedIndex = counter;
                if (currValue == ddl.SelectedItem.ToString())
                {
                    ddl.Items.RemoveAt(counter);
                    counter = counter - 1;
                }
            }
        }
    }

}