switch (ddlLanden.SelectedValue)
{
    case "1":
        ddlProvincies.Items.Add(new ListItem("Groningen", "1"));
        ddlProvincies.Items.Add(new ListItem("Friesland", "2"));
        ddlProvincies.Items.Add(new ListItem("Drenthe", "3"));
        ddlProvincies.Items.Add(new ListItem("Overijssel", "4"));
        ddlProvincies.Items.Add(new ListItem("Gelderland", "5"));
        ddlProvincies.Items.Add(new ListItem("Flevoland", "6"));
        ddlProvincies.Items.Add(new ListItem("Zuid-Holland", "7"));
        ddlProvincies.Items.Add(new ListItem("Noord-Holland", "8"));
        ddlProvincies.Items.Add(new ListItem("Utrecht", "9"));
        ddlProvincies.Items.Add(new ListItem("Zeeland", "10"));
        ddlProvincies.Items.Add(new ListItem("Noord-Brabant", "11"));
        ddlProvincies.Items.Add(new ListItem("Limburg", "12"));
        break;

    case "2":
        ddlProvincies.Items.Add(new ListItem("Antwerpen", "1"));
        ddlProvincies.Items.Add(new ListItem("West-Vlaanderen", "2"));
        ddlProvincies.Items.Add(new ListItem("Oost-Vlaanderen", "3"));
        ddlProvincies.Items.Add(new ListItem("Limburg", "4"));
        ddlProvincies.Items.Add(new ListItem("Vlaams-Brabant", "5"));
        ddlProvincies.Items.Add(new ListItem("Luik", "6"));
        ddlProvincies.Items.Add(new ListItem("Luxemburg", "7"));
        ddlProvincies.Items.Add(new ListItem("Namen", "8"));
        ddlProvincies.Items.Add(new ListItem("Waals-Brabant", "9"));
        ddlProvincies.Items.Add(new ListItem("Henegouwen", "10"));
        break;

    default:
        break;
}