用js得到select被选中的值

<script language="javascript">
 function show(obj){
  alert(obj);
 }
</script>

<form id="form1" name="form1" method="post" action="">
  <select name="ss" id="ss"  onchange="show(this.options[this.options.selectedIndex].value)">
    <option value="z">1</option>
    <option value="t">2</option>
    <option value="s">3</option>
    <option value="d">4</option>
  </select>