关于3级选择的问题

给你段代码试一下：
<TITLE>三级联动</TITLE>
<STYLE>
body { font-size: 11px; font-family: Verdana;background:#ececec;color:#666666;}
select { font-size: 11px; font-family: Verdana;vertical-align: middle;margin: 3px;background:#ececec;color:#666666;}
</STYLE>
<div id="tar">&nbsp;</div>
<script>var createDate = "2003.8.19";</script>
<SCRIPT LANGUAGE="JavaScript" DEFER>
var fMenu = ["上海","北京","广东","南京"]
var fValue = ["shanghai","beijing","guadong","nanjing"]
var sMenu = [["杨浦区","徐汇区","黄浦区","卢湾区"],["丰台区","宝定区"],["广州","深圳"],["雨花台","郊区别墅"]]
var sValue = [["yp","xh","hp","lw"],["ft","bd"],["gz","sz"],["yht"]]
var tMenu = [[["市光新村","工农三村"],["徐汇新村","徐汇高楼"],["黄浦楼宇","外滩风景"],["卢湾菜场","卢湾体育馆"]],[["亚运村","无名村"],["宝定村"]],[["广州市区","广州郊区"],["深圳市区","福田"]],[["南京市区","南京郊区"],["别墅1","别墅2"]]]var oWhere = document.all.tar;
var ofMenu = document.createElement("<SELECT name='city'>");
var osMenu = document.createElement("<SELECT name='region'>");
var otMenu = document.createElement("<SELECT name='village'>");
with(oWhere)appendChild(ofMenu),appendChild(osMenu),appendChild(otMenu);createMainOptions();
createSubOptions(0);
createSub2Options(0,0);ofMenu.onchange = function() {createSubOptions(this.selectedIndex);createSub2Options(this.selectedIndex,osMenu.selectedIndex);};
osMenu.onchange = function() {createSub2Options(ofMenu.selectedIndex,this.selectedIndex);};function createMainOptions() {
    for(var i=0;i<fMenu.length;i++)ofMenu.options[i] = new Option(fMenu[i],fValue[i]);
}
function createSubOptions(j) {
    with(osMenu) {
        length=0;
        for(var i=0;i<sMenu[j].length;i++)osMenu.options[i] = new Option(sMenu[j][i],sValue[j][i]);
    }
}
function createSub2Options(j,k) {
    with(otMenu) {
        length=0;
        for(var i=0;i<tMenu[j][k].length;i++)otMenu.options[i] = new Option(tMenu[j][k][i]);
    }
}
</SCRIPT>