为什么实现不了
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>菜单</title>
<link rel="stylesheet" type="text/css" href="cd.css" />
<script type="text/javascript" >function showsubmenu(li){
var a=li.document.getElementsByTagName("ul")[0];
a.style.display="block";
}
function hidesubmenu(li){
var b=li.document.getElementsByTagName("ul")[0];
b.style.display="none";}
</script>
</head>
<body>
<div id="cd">
<ul><li><a href="#">首页</a></li>
<li onmouseover="showsubmenu(this)" onmouseout="hidesubmenu(this)">
<a href="#">交流中心</a><ul><li>贴吧</li><li>微博</li></ul>
</li>
<li><a href="#">微博</a></li>
<li><a href="#">产品展示</a></li>
<li><a href="#">团队</a></li>
</ul>
</div>
