Js Object Operation

<select onchange='ok(this)' id=a><option>音乐<option>电影</select>
<select id=b>
</select>
<script>
function ok(objs){
if(objs.options[objs.selectedIndex].text=="音乐")b.outerHTML="<select id=b><option selected>音乐1<option>音乐2"
if(objs.options[objs.selectedIndex].text=="电影")b.outerHTML="<select id=b><option selected>电影1<option>电影2"
}
ok(a)
</script>