用js做省市级联菜单，用二维数组实现，不知为何就是无法实现，求大神点拨，下面是代码：

<form name="form1" method="post" action="">
 
  <select   id="sheng" onChange="changecity();"><script type="text/javascript">fillsheng();</script>
  </select>
  <select id="shi">
  <option value="0000">--请选择城市--</option>
  </select>
</form>
***********js代码
var provinces = new Array(
	new Array("01","北京"),
	new Array("02","安徽"),
	new Array("03","湖北"),
	new Array("04","江苏")

	);
	var cities = new Array(
	new Array("0102","海淀"),
	new Array("0103","朝阳"),
	new Array("0202","芜湖"),
	new Array("0204","马鞍山"),
	new Array("0301","武汉"),
	new Array("0303","荆州"),
	new Array("0401","南京"),
	new Array("0402","苏州")

	);
function fillsheng(){

	var sheng = document.getElementById("sheng");
	for(var i=0;i<provinces.length;i++){
		var option = new Option(provinces[i][1],provinces[i][0]);
		sheng.add(option,null);
		}
		sheng.options[0].selected = true;
	
	}

function changecity(){
	var sheng = document.getElementById("sheng");
	var procode = sheng.value;
	var city = document.getElementById("shi");
	city.options.length = 0;
	city.add(new Option("选择城市","0000"),null);
	for(var i=0;i<cities.length;i++){
		if(cities[i][0].substring(0,2) == procode)
		city.add(new Option(cities[i][1],cities[i][0]),null);
		city.options[0].selected = true;	
		}
	}