下拉菜单不显示啊，这是为什么？
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Hello World</title>
<link href="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div>
<nav class="navbar navbar-default">
<div>
<!-- Brand and toggle get grouped for better mobile display -->
<div>
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
<span>Toggle navigation</span>
<span></span>
<span></span>
<span></span>
</button>
<a href="#">某管理系统</a>
</div>
<!-- Collect the nav links, forms, and other content for toggling -->
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li><a href="#">首页 <span>(current)</span></a></li>
<li>
<a href="#" data-toggle="dropdown" role="button" aria-expanded="false">功能 <span></span></a>
<ul role="menu">
<li><a href="#">业务功能</a></li>
<li><a href="#">信息建立</a></li>
<li><a href="#">信息查询</a></li>
<li><a href="#">信息管理</a></li>
<li></li>
<li><a href="#">系统功能</a></li>
<li><a href="#">设置</a></li>
</ul>
</li>
<li><a href="#">帮助</a></li>
</ul>
<form class="navbar-form navbar-right">
<div>
<input type="text" placeholder="用户名...">
<input type="text" placeholder="密码...">
</div>
<button type="submit" class="btn btn-default">登录</button>
</form>
</div><!-- /.navbar-collapse -->
</div><!-- /.container-fluid -->
</nav>
</div>
<link href="http://cdn.bootcss.com/jquery/1.11.1/jquery.min.js" rel="stylesheet" type="text/javascript" />
<link href="http://cdn.bootcss.com/bootstrap/3.2.0/js/bootstrap.min.js" rel="stylesheet" type="text/javascript" />
</body>
</html>
