一模一样的代码，得出的效果不一样，搞不懂是为什么
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>为导航条添加标题、二级菜单及状态</title>
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<script src="http://cdn.bootcss.com/jquery/3.1.1/jquery.min.js"></script>
<script src="http://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.js"></script>
</head>
<body>
<!--加入导航条标题-->
<div class="navbar navbar-default" role="navigation">
<div class="navbar-header">
<a href="##" class="navbar-brand">慕课网</a>
</div>
<ul class="nav navbar-nav">
<li class="active"><a href="##">网站首页</a></li>
<li><a href="##">系列教程</a></li>
<li><a href="##">名师介绍</a></li>
<li><a href="##">成功案例</a></li>
<li><a href="##">关于我们</a></li>
</ul>
</div>
<!--导航条状态及二级菜单-->
<div class="navbar navbar-default" role="navigation">
<div class="navbar-header">
<a href="##" class="navbar-brand">慕课网</a>
</div>
<ul class="nav navbar-nav">
<li class="active"><a href="##">网站首页</a></li>
<li class="dropdown">
<a href="##" data-toggle="dropdown" class="dropdown-toggle">系列教程<span class="caret"></span></a>
<ul class="dropdown-menu">
<li><a href="##">CSS3</a></li>
<li><a href="##">JavaScript</a></li>
<li class="disabled"><a href="##">PHP</a></li>
</ul>
</li>
<li><a href="##">名师介绍</a></li>
<li><a href="##">成功案例</a></li>
<li><a href="##">关于我们</a></li>
</ul>
</div>
<div class="navbar navbar-default" role="navigation">
<div class="navbar-header">
<a href="##" class="navbar-brand">慕课网</a>
</div>
<ul class="nav navbar-nav">
<li class="active"><a href="#">网站首页</a></li>
<li><a href="#">名师介绍</a></li>
<li><a href="#">成功案例</a></li>
<li class="dropdown">
<a href="#" data-toggle="dropdown" class="dropdown-toggle">
关于我们
<span class="caret"></span>
</a>
<ul class="dropdown-menu">
<li><a href="#">公司简介</a></li>
<li class="disabled"><a href="#">企业文化</a></li>
<li><a href="#">组织结构</a></li>
<li><a href="#">客服服务</a></li>
</ul>
</li>
</ul>
</div>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script src="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
</body>
</html>
看图上显示的结果
