Well,
there are 2 Approaches:
contol: asp drop down list.
event: protected void drop_downList1_SelectedIndexChanged(object sender, EventArgs e)
{
}
protected void drop_downList1_SelectedIndexChanged(object sender, EventArgs e)
{
string selectedSub = ddlRepetsub1.SelectedItem.Text;
drop_downList2.Items.FindByText(selectedSub).Enabled = false;
}
protected void drop_downList1_SelectedIndexChanged(object sender, EventArgs e)
{
string selectedSub = ddlRepetsub1.SelectedItem.Text;
drop_downList2.Items.Remove(selectedSub);
}