Dependent Dropdownlist boxes

I have a form with two drop down boxes. The first one, dropCounty is fine. But I'd like for the second one, dropCity to depend on dropCounty for the value. I would like for the dropCity box to show only cities that are for the certain county that was chosen. I would like for the second drop down box to be hidden until the person has chosen a county.  Can someone please help me with the code for this?


Here's my code:

        '----------------------------------------------------------------

        ' name: BindCountyList() - Fine

        '----------------------------------------------------------------

        Sub BindCountyList()

    Dim strConnect As String   

    Dim conCounty As New System.Data.SqlClient.SQLConnection

    Dim cmdSelect As New System.Data.SqlClient.SQLCommand

    Dim dtrCounty As System.Data.SqlClient.SqlDataReader

    'Get connection string from Web.Config

    strConnect = ConfigurationSettings.AppSettings("ConnectionString")

    conCounty = New System.Data.SqlClient.SQLConnection(strConnect)

    conCounty.Open()

    cmdSelect = New System.Data.SqlClient.SQLCommand( "Select CountyID, County From County Order by County", conCounty)

    dtrCounty = cmdSelect.ExecuteReader()

    dropCounty.DataSource = dtrCounty

    dropCounty.DataValueField = "CountyID"

    dropCounty.DataTextField = "County"

    dropCounty.DataBind()

     'Add and select a "Select County" option for the DDL

     dropCounty.Items.Insert(0, new ListItem("Select County", ""))

     dropCounty.SelectedIndex = 0

    dtrCounty.Close()

  &nbs