关于多选后所对应的值

我该如何获取选择选择框所对应后面的下拉框的值图片说明小白求救啊 刚刚开始学

 <html>
<head>
<script type="text/javascript">
function aaa()
{
alert(document.getElementById("mySelect").value)
}
</script>
</head>
<body>

<form>
<select id="mySelect" onchange="aaa()">
  <option>苹果</option>
  <option>桃子</option>
  <option>香蕉</option>
  <option>桔子</option>
</select>

</form>

</body>
</html>