Combobox initial Value

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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
namespace Combobox
{
    public partial class Form1 : Form
    {
        string ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
        OleDbCommand com;
        OleDbDataAdapter oda;
        DataSet ds;
        string str;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.Add("Choose CourseID");
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Courses";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox1.Items.Add(reader["CourseID"]); 
            }
          
           
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            comboBox2.Items.Clear();
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Students where CourseID='" + comboBox1.Text.Trim() + "'";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox2.Items.Add(reader["FirstName"].ToString());
            }
            con.Close();
            reader.Close();
           
        }
    }
}