Extjs ajax 写后台程序用asmx文件，以次是代码？出错时找不到正确的Url

Extjs ajax 写后台程序用asmx文件，以下是代码？出错时找不到正确的Url
前台Js： 
<script type="text/javascript">
        Ext.onReady(function () {
            var button = new Ext.Button({
                renderTo: "div1",
                text: "后台Ajax提交",
                handler: function () {
                    Ext.Ajax.request({
                        url: 'WebService1.asmx/HelloWorld',
                        headers: {
                            'Content-Type': 'application/json; charset=utf-8'
                        }, 
//                        headers: {
//                            'userHeader': 'userMsg'
//                        },
                        params: { a: 10, b: 20 },
                        method: 'GET',
                        success: function (response, options) {
                            Ext.MessageBox.alert('成功', '从服务端获取结果: ' + response.responseText);
                        },
                        failure: function (response, options) {
                            Ext.MessageBox.alert('失败', '请求超时或网络故障,错误编号：' + response.status);
                        }
                    });
                },
                id: "bt1"
            });

        });  
    </script>

后台asmx： public class WebService1 : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld()
        {
            return "Hello World";
        }
    }



请高手指教啊
------解决方案--------------------
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
//若要允许使用 ASP.NET AJAX 从脚本中调用此 Web 服务，请取消对下行的注释。 
[System.Web.Script.Services.ScriptService]

把第四行的注释去掉就行了，20分给我吧