联动菜单实现思路

1: 选择select时,如何触发? 哪种事件? onchange
2: 如何获取被选中的option的值 ? selectobj.value
3: 动态生成option innerHTML

<select name="add" onchange="change()">
        <option value="">请选择</option>
        <option value="0">北京</option>
        <option value="1">安徽</option>
    </select>
    <select name="city" id=""></select>
    <script type="text/javascript">
         function change() {
            var area = [ ['朝阳' , '海淀' , '昌平'] , ['淮北','淮南' , '铜陵'] ];
            var s1 = document.getElementsByName("add")[0];
            var city = document.getElementsByName('city')[0];
            if(s1.value=="") {
                city.innerHTML = '';
                return;
            } else {
                for(var i=0,str='',len=area[s1.value].length;i<len;i++) {
                    str += '<option value='+i+'>'+area[s1.value][i]+'</option>';
                    city.innerHTML = str;
                }
            }

         }

    </script>