ExecuteQuery vs ExecuteReader

I don't know why when I use select with ExecuteQuery my result will have repeated the lastes line.
And my friend adviced me that I should use ExecuteReader now it's work
What's the different ???

here is my code !!

public void Add()
        {
            if (File.Exists("" + info + ".db"))
            {
                sql_con = new SQLiteConnection("Data Source= " + info + ".db;Version=3;New=False;Compress=True;");
                sql_con.Open();
                sql_cmd = sql_con.CreateCommand();
                sql_cmd.CommandText = "drop table par ;";
                sql_cmd.ExecuteNonQuery();
            }
            else
            {
                sql_con = new SQLiteConnection("Data Source= " + info + ".db;Version=3;New=True;Compress=True;");
                sql_con.Open();
                sql_cmd = sql_con.CreateCommand();
            }
            sql_cmd.CommandText = "create table par (Date nvarchar(20), Time nvarchar(10), Linein NVARCHAR(20), Lineout NVARCHAR(20), Number NVARCHAR(20), Status nvarchar(10),Duration int(20), Cost nvarchar(20), Detail nvarchar(10), Network nvarchar(10));";
            sql_cmd.ExecuteNonQuery();
            StreamReader reader = new StreamReader(inputfilename);
            string line = reader.ReadLine();
            while (line != null)
            {
                string NNDuration = line.Replace("'", "''");
                string Date = line.Substring(12, 8);
                if (line[11] == '=')
                {
                    string Time = line.Substring(21, 5);
                    string Linein = line.Substring(27, 10);
                    string Lineout = line.Substring(38, 1);
                    if (string.Compare(Lineout, "0") >= 0 && string.Compare(Lineout, "9") <= 0)
                    {
                        string Number = line.Substring(40, 20);
                        string Status = line.Substring(61, 3);
                        string NDuration = NNDuration.Substring(65, 10);
                        string b = NDuration.Substring(2, 2);
                        string c = NDuration.Substring(6, 2);
                        int y = Convert.ToInt32(b);
                        int l = Convert.ToInt32(c);
                        int Duration = y + l;
                        string OCost = line.Substring(73, 5);
                        string Cost = OCost.Trim();
                        string Detail = line.Substring(79, 3);
                        string Network = line.Substring(82, 8);
                        sql_cmd.CommandText = "insert into par values('" + Date + "','" + Time + "','" + Linein + "','" + Lineout + "','" + Number + "','" + Status + "','" + Duration + "','" + Cost + "','" + Detail + "','" + Network + "');";
                        sql_cmd.ExecuteNonQuery();
                      
                        textBox14.Text = textBox14.Text + "\r\n" + Date + " ";
                    }
                    else
                    {
                        textBox5.Text = textBox5.Text + "\r\n" + line;
                    }
                }
                else
                {
                    textBox5.Text = textBox5.Text + "\r\n" + line;
                }
               line = reader.ReadLine();
            }
           
        }
private void LoadData()
        {
            string CommandText = "select * from par "; //??????????????
            sql_cmd.ExecuteReader();
            DB = new SQLiteDataAdapter(CommandText, sql_con); //???????????????????????????????????
            DS.Reset(); //?? dataset ????????????????? reset ????????????
            DB.Fill(DS); //?????????????????????????????????????????????? ????????? Dataset
            DT = DS.Tables[0]; //??????????????????????????????? datatable
            dataGridView1.DataSource = DT; //???????? datatable ??????????????? dataGridViews
            sql_con.Close();
          
        }