drop down list problem

i have two drop down list and radio button.
Positions : dropdown list
Status : Open
Close
Vacancies : dropdownlist

my prob is i select positions dropdown list values is 20 total(50) after i select status open , then vacancies should be visible to 20 only total(50).


my code,

if (rd_status.SelectedValue == "open")
{
// ddlpositions.SelectedValue = ddlvacc.SelectedValue;
//ddposisions.SelectedValue = lblvacancis.Text;
ddlvacc.Text = ddlpositions.SelectedValue;
}
else if (rd_status.SelectedValue == "close")
{
// ddposisions.SelectedValue = lblvacancis.Text;
ddlvacc.Text = ddlpositions.SelectedValue;
}
}

but it works only when i take textbox in vacancies feild , i want dropdown list , if any body know pls send me solution