string sql = "SELECT posicao FROM passageiros; ";
                SqlCommand exeSql = new SqlCommand(sql, cn);
                cn.Open();
                SqlDataReader reader = exeSql.ExecuteReader();
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        assentos.Add(reader.GetString(0));

                    }
                }