How to populate selected country states only in asp.net on running time

Protected void ddlCtry_OnSelectedIndexChanged(object sender, EventArgs e)
{
            DataSet ds=//get data from database by passing ctry id as input parameter
            ddlState.DataSource=ds;
           ddlState.DataTextField="StateName";
           ddlState.DataValueField="StateId";
           ddlState.DataBind();
}

Private Sub frm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        populateCountries(frm_cmbCountry)
End Sub

 Private Sub populateCountries(ByVal cmb As ComboBox)
        Try
            Dim dsCountries As DataSet = GetcountryNamesUsingQuery()
            If dsCountries.Tables(0).Rows.Count > 0 Then
                cmb.DataSource = dsCountries.Tables(0).DefaultView
            End If
            cmb.SelectedIndex = -1
        Catch ex As Exception

        End Try
    End Sub

Private Sub frmcmbCountry_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles frmcmb.SelectedIndexChanged
                      //call PopulateStates method
       End Sub

protected void page_Load(object sender, EventArgs e)
{
  if(!IsPostBack)
  {
   // Load Countries
   LoadCountries();
  }
}
protected void ddlCountry_OnSelectedIndexChanged(object sender, EventArgs e)
{
  // Load all the states based on the selected country value
  LoadStates(ddlCountry.SelectedItem.Text);
}
// In the same way, we can do for the cities.
protected void ddlState_OnSelectedIndexChanged(object sender, EventArgs e)
{
  // Load all the cities based country and state selected value
  LoadCities(ddlCountry.SelectedItem.Text , ddlState.SelectedItem.Text);
}