private void button2_Click(object sender, EventArgs e)
        {
            lb_cuorse.Items.Clear();
            CoursesDB     C             = new CoursesDB();
            List <string> listOfCourses = new List <string>();

            string semester = returnSemestr(cb_semester.SelectedItem.ToString());// SelectedItem.ToString();
            string year     = returnYear(cb_year.SelectedItem.ToString());

            listOfCourses = C.fillCoursesBySenester_list(semester, year);

            foreach (string Item in listOfCourses)
            {
                lb_cuorse.Items.Add(Item);
            }
        }