二级联动的问题

<script language="Javascript">
<!--
var city = new Array(
new Array("浦东新区-浦东新区","徐汇区-徐汇区","长宁区-长宁区","普陀区-普陀区","闸北区-闸北区","虹口区-虹口区","杨浦区-杨浦区","黄浦区-黄浦区","卢湾区-卢湾区","静安区-静安区","宝山区-宝山区","闵行区-闵行区","嘉定区-嘉定区","金山区-金山区","松江区-松江区","青浦区-青浦区","崇明县-崇明县","奉贤区-奉贤区","南汇区-南汇区"),
new Array("合肥-合肥","芜湖-芜湖","蚌埠-蚌埠","巢湖-巢湖","滁州-滁州","淮南-淮南","安庆-安庆","池州-池州","宣城-宣城","马鞍山-马鞍山","黄山-黄山","铜陵-铜陵","宿州-宿州","淮北-淮北","阜阳-阜阳","其它-其它"),
new Array("北京-北京","其它-其它")
);function test(a,b){
var index = a.options.selectedIndex-1;
b.length = city[index].length;
for(var i = 0;i<city[index].length;i++)
{
var text = city[index][i].split("-");
b.options[i].text = text[0];
b.options[i].value =text[1];
}
}
//-->
</script>
<select name="LicenceType" onChange=test(LicenceType,b)> 
<option>--选择城市--</option>
<option value="aa">aa</option> 
<option value="bb">bb</option> 
<option value="cc">cc</option> 
</select> 
<select name="b">
<option>--选择地区--</option>
</select>