实现三级联动，很多网站都有例如：“省-市-县”，实现省-市-县的三级联动的效果，怎么弄？
去这里下载我刚上传上去
http://download.csdn.net/source/1665868
HTML code:
<html>
<script type="text/javascript">
var citys=[
["西安","咸阳","宝鸡","渭南","汉中"],
["深圳","广州","中山","东莞","惠州","佛山","珠海"],
["上海"]
];
function getCity()
{
var sltProvince=document.getElementById("province");
var sltCity=document.getElementById("city");
sltCity.options.length=1;
if(sltProvince.selectedIndex>0)
{
var provinceCity=citys[sltProvince.selectedIndex-1];
for(var i=0; i<provinceCity.length; i++)
{
var city=new Option(provinceCity[i],i);
sltCity.options.add(city);
}
}
}
</script>
<body>
<select name="province" id="province" onchange="getCity();" >
<option value="0">请选择省份</option>
<option value="1">陕西</option>
<option value="2">广东</option>
<option value="3">上海</option>
</select>
<select name="city" id="city">
<option value=&qu
