问使用asp WebMethod中的数据填充jquery自动完成

EN

[WebMethod]
public static string GetCities(string cityName)
{
    JArray json = new JArray(
        new JObject(new JProperty("label", "label1"), new JProperty("category", "cat1"), new JProperty("value", "v1")),
        new JObject(new JProperty("label", "label2"), new JProperty("category", "cat1"), new JProperty("value", "v2")),
        new JObject(new JProperty("label", "label3"), new JProperty("category", "cat2"), new JProperty("value", "v3")),
        new JObject(new JProperty("label", "label4"), new JProperty("category", "cat3"), new JProperty("value", "v4")));
    return json.ToString();
}

<script type="text/javascript">
    $(function () {
        $("#txtCity").autocomplete({
            source: function (request, response) {
                var param = { cityName: $('#txtCity').val() };
                $.ajax({
                    url: "WebForm1.aspx/GetCities",
                    data: JSON.stringify(param),
                    dataType: "json",
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    dataFilter: function (data) { return data; },
                    success: function (data) {
                        response(data.d); //here data.d contains the json array string
                    },
                    error: function (XMLHttpRequest, textStatus, errorThrown) {
                        alert(textStatus);
                    }
                });
            },
            select: function( event, ui ) {
                $( "#output" ).val( ui.item.value );
                return false;
            },
            minLength: 2//minLength as 2, it means when ever user enter 2 character in TextBox the AutoComplete method will fire and get its source data. 
        });
    });
</script>