Its workin :
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
if ("Graduation" == comboBox1.SelectedItem.ToString())
{
comboBox2.Items.Add("BBM");
comboBox2.Items.Add("BCA");
}
if ("Post-Graduation" == comboBox1.SelectedItem.ToString())
{
comboBox2.Items.Clear();
comboBox2.Items.Add("MA");
comboBox2.Items.Add("MBA");
comboBox2.Items.Add("MCA");
}
}