private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Grups g = new Grups();
g.OpenConnection(connectionString);
int idgr = g.GetIdByNzach(nzach);
g.CloseConnection();
Predmets p = new Predmets();
p.OpenConnection(connectionString);
int idprd = p.GetIdByName(comboBox1.Text);
p.CloseConnection();
Marks m = new Marks();
m.OpenConnection(connectionString);
dataGridView1.DataSource = m.GetMarksByGroupAndPredmet(idgr, idprd);
m.CloseConnection();
}