CascadingDropDownBox

Hi, I have created a webpage with two drop down boxes.

Continent: <asp:DropDownList ID="ddlContinent" runat="server" /><ajaxToolkit:CascadingDropDown ID="cddcontinent" runat="server" TargetControlID="ddlContinent"
    Category="Continent" ServicePath="Helper.asmx" ServiceMethod="GetContinents" /><br />

Country: <asp:DropDownList ID="ddlCountry" runat="server" /><ajaxToolkit:CascadingDropDown ID="ccdCountry" runat="server" TargetControlID="ddlCountry"
    ParentControlID="ddlContinent" Category="country"
    ServicePath="Helper.asmx" ServiceMethod="GetCountries" /><br />

I've also created a webservice (section of it below)

<WebMethod> _
    Public Function GetContinents(knownCategoryValues As String, category As String) As CascadingDropDownNameValue()

        Dim Sql As Object

        Dim MyConnection = New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;  data source=" & Server.MapPath("\App_data\MyDayabase.mdb"))
        MyConnection.Open()

        Sql = " SELECT ContinentName, ContinentID FROM Continents "

        Dim dbcomm As New OleDbDataAdapter(Sql, MyConnection)
        Dim values As New List(Of CascadingDropDownNameValue)()

        Dim dt As New DataTable()
        dbcomm.Fill(dt)

        For Each dr As DataRow In dt.Rows
            Dim continent As String = DirectCast(dr("ContinentName"), String)
            Dim continentId As Integer = CInt(dr("ContinentID"))
            values.Add(New CascadingDropDownNameValue(continent, continentId.ToString()))
        Next

        Return values.ToArray()

    End Function

What I have been really struggling with though is the second part of the webservice to populate the second drop box.

I have two tables in my database

Continents

ContinentID, ContinentName
1, Europe
2, Asia

Countries

CountryID, ContinentID, CountryName
1, 1, England
2, 1, France
3, 2, Japan
4, 2, China 

Please could someone help as to what I should put for GetCountries

Thanks Very Much