Ajax省市联动

以JQuery为JS，写的Ajax省市联动。
代码如下:
<!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/xhtml">
<head>
    <title>无标题页</title>

    <script src="Script/jquery-1.10.2.js" type="text/javascript"></script>

    <script src="Script/Common.js" type="text/javascript"></script>

    <script type="text/javascript">
    var xhr=false;
    var proSel=false;//selPro
    var citySel=false;//selCity
    var Pid=false;
    $(function(){
        
        xhr=createXhr();
        
        proSel=$('#selPro');
        citySel=$('#selCity');
        
        //加载省下拉框数据
        getAreaData(0);
       
        //省下拉框发生改变时，加载市下拉框数据     
        proSel.change(function(){
            Pid=$(this).val();
            getAreaData(Pid);
        });
        
    });


    //1.
    function getAreaData(pId){
        //1.open
        xhr.open('get','1-getAreaData.ashx?pid='+pId,true);
        //2.set
        xhr.setRequestHeader('If-Modified-since',0);
        //3.onreadystatechange
        xhr.onreadystatechange=function(){
        
        if(xhr.readyState==4){
                if(xhr.Status==200){
                    var res=xhr.responseText;
                    if (pId==0){
                        data2Sel(res,proSel);
                        //首次加载时，加载市下拉框数据
                        pId=$(':selected').val();
                        getAreaData(pId);
                    }else{
                    data2Sel(res,citySel);                    
                    }
                    
                }
            }
        };
        
        xhr.send();
    }
    
    
    //2.
    function data2Sel(data,selOption){
        selOption.empty();
        
        var res=eval(data);
        
        for(var i=0;i<res.length;i++){
            var opt=false;
            if(i==2){
            opt=$('<option value='+res[i].Id+' selected=true>'+res[i].Name+'</option>');
            }else{
             opt=$('<option value='+res[i].Id+'>'+res[i].Name+'</option>');
            }
            selOption.append(opt);
        }
    }
    
    
    </script>

</head>
<body>
    <select id="selPro">
    </select>
    <select id="selCity">
    </select>
</body>
</html>