asp.net ajax WebService.asmx

js代码不能将数据发送到WebService1.asmx中对应的方法，只能进入insertCallbackSelsectSma提示错误的函数，以下是代码
js代码：
 function GetClassByItemIdcy(id) {
                 var data = "{id:" + parseInt(id) + "}";
                 //alert(data);
                 $.ajax({
                     type: "POST",
                     contentType: "application/json; charset=utf-8",
                     url: "WebService1.asmx/SelectAllClassByItemId",
                     data: data,
                     dataType: "json",
                     success: insertCallbackSelsectSma,
                     error: errorCallbackSelsectSma
                 });
             }
             function insertCallbackSelsectSma(result) {
                 $("#DropDownList1").find("option").remove();
                 var i = result.d;
                 //alert(i);
                 if (result["d"].length > 0) {
                     for (var i = 0; i < result["d"].length; i++) {
                         var classsma = eval(result["d"][i]);
                         alert(classsma.Class_Id);
                         $("#DropDownList1").append('<option value="' + classsma.ClassId + '">' + classsma.ClassName + '</option>');
                     }
                 }
             }
             function errorCallbackSelsectSma(XMLHttpRequest, textStatus, errorThrown) {
                 alert("错误1");
             }页面中的dropdownlist        &nbsp;&nbsp;<asp:DropDownList ID="DropDownList2" runat="server" onchange="GetClassByItemIdcy(this.value)">WebService1.asmx 中的代码
[WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
        public List<ClassEntity> SelectAllClassByItemId(int id)
        {
            classbllall = new ClassBll();
            return classbllall.ClassSelectIdById(id);
        }