AJAX WS Call return result problem

Hi,

 I am pretty new using AJAX to call WS.First I have some code like this.

 In ServerTest.asmx it is just the simple HelloWorld return string call.

And I do enable  [System.Web.Script.Services.ScriptService] in asmx. 
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="MySite._Default" %>

<%@ Register Assembly="AjaxControlToolkit, Version=1.0.10920.26879, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e"
Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xh ...