各位帮帮忙，我有个问题ajax的，谢谢

function DptBindXML(as_Url,as_Value,as_Text,ab_IsCom,ab_Asyn)
{
   var dpt = $("OrgDpt1_DDL_Depart");    //获取部门下拉列表框
   // 清除原有数据
   var li_DdlLen = dpt.options.length;
　 for(var i = 0;i < li_DdlLen; i++)
　 {
　  　dpt.remove(0);
　 }
　
　 // 增加一个选择项
　 var newOption = document.createElement("OPTION");
　 newOption.text = " -- 请选择 -- ";
　 newOption.value = "";
　 dpt.options.add(newOption);
　
　 //如果机构中选中的是  -- 请选择 --  ，则在部门中只出现 -- 请选择 --
　 if ($("OrgDpt1_DDL_Organ").value == "请选择")    //$("OrgDpt1_DDL_Organ")是机构下拉列表框
　 {
　      var dpt1 = $("OrgDpt1_DDL_Depart");
　      for(var i = 0;i < dpt1.options.length ; i++ )
　      {
　  　      dpt1.remove(i);
　      }
　      var newOption1 = document.createElement("OPTION");
　      newOption1.text = "-- 请选择 --";
　      newOption1.value = "";
　      dpt1.options.add(newOption1);
　      return;
　 }
　
　 var xmlHttp = CreateXMLHttp();
　 xmlHttp.open("Post",as_Url,ab_Asyn);   　
   xmlHttp.setRequestHeader ("Content-Type","text/plain; charset=utf-8");
   xmlHttp.onreadystatechange=function()
   {      
        if(xmlHttp.readyState == 4)
        {
            if (xmlHttp.status == 200)              //异常了500？？？不知道为什么这里是500，我直接执行获取数据的页面都能得到数据，我找不到原因
            {  
                 var oXmlData = xmlHttp.responseXML.getElementsByTagName("DocumentElement")[0];    //这里也是，获取不到数据，不知道为什么
                 if (oXmlData == null )
                 {
                    xmlHttp = null;
                    delete xmlHttp;
                    return;
                  }                 
                 
                 for(var i = 0; i < oXmlData.childNodes.length; i++ )
                 {  
                    newOption = document.createElement("OPTION");
                  
                    //处理显示效果
                    var blank;
                    var j = oXmlData.getElementByTagName("Grade")[i].firstChild.nodeValue;
                    for (var k=2; k<j; k++)
                    {
                        blank += "&nbsp;&nbsp;&nbsp;";
                    }
                    blank += "&nbsp;├&nbsp;";
                  
                    if(ab_IsCom)
                    {
                        newOption.text ="["+ oXmlData.getElementsByTagName(as_Value)[i].firstChild.nodeValue+"] " + Server.HtmlDecode(blank + oXmlData.getElementsByTagName(as_Text)[i].firstChild.nodeValue);
                    }
                    else
                    {
                        newOption.text = oXmlData.getElementsByTagName(as_Text)[i].firstChild.nodeValue;
                    }
                    newOption.value = oXmlData.getElementsByTagName(as_Value)[i].firstChild.nodeValue;
                    dpt.options.add(newOption);
                 }
              xmlHttp = null;
              delete xmlHttp;
              return true;
            }
        }
    }
    xmlHttp.send(null);
}
</script>