_XMLHTTP = ["Msxml2.XMLHTTP.6.0", "Msxml2.XMLHTTP.5.0", "Msxml2.XMLHTTP.4.0", 
                "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP"];
function createXMLHttp(){
    var rtn = null ;
    if ( window.XMLHttpRequest ) {
        rtn = new XMLHttpRequest();
    }else if ( window.ActiveXObject ){
        rtn = newActiveXObject();   
    }
    return rtn ;
}
function newActiveXObject(){    
    var rtn = null ;
    for (var i = 0; i < _XMLHTTP.length; i++) {
        try {
            rtn = new ActiveXObject(_XMLHTTP[i]);
            astr = _XMLHTTP[i] ;
            break;
        }catch (ex) { /* ignore */ }
    }
    return rtn; 
}


function  addOption(objSelect,v,t){
    var pos = objSelect.selectedIndex;
    var objOption = new Option(t, v);
    var fieldExist = false;
    var i = 0;
    for (i=0;i<objSelect.options.length;i++) { 
        if (objSelect.options[i].value == v) { 
            fieldExist = true;
            alert("“"+t+"”已经存在");
            break; 
        } 
    } 
    if (!fieldExist) { 
        if (pos == -1||pos > objSelect.options.length){
            objSelect.options[objSelect.options.length] = objOption;
        }else{
            //objSelect.add(objOption, pos);
            objSelect.options.add(objOption, pos);
            objSelect.options[pos].selected=true;
        } 
    } 
    for (i=0; i<objSelect.options.length; i++){
        objSelect.options[i].selected=false;
    } 
}