html – Bootstrap导航栏切换不起作用

使用下面的代码我试图创建一个响应式Web布局,在减小屏幕大小时,将显示一个导航栏切换按钮,并在选择时将列出页面导航.我成功地显示了切换按钮,但是在点击时,没有任何反应.我已经回顾了各种Bootstrap示例,并且无法确定我的失败原因.
任何援助将不胜感激.

<nav class="navbar navbar-inverse navbar-fixed-top">
      <div class="container">
        <div class="navbar-header">
          <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
        <span class="icon-bar"></span>
          </button>

     <div id="navbar" class="navbar-collapse collapse">
          <ul class="nav navbar-nav">
            <li class="active"><a href="#">Home</a></li>
            <li><a href="#about">About</a></li>
            <li><a href="#contact">Contact</a></li>
            <li class="dropdown">
              <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-expanded="false">Dropdown <span class="caret"></span></a>
              <ul class="dropdown-menu" role="menu">
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
                <li><a href="#">Something else here</a></li>
                <li class="divider"></li>
                <li class="dropdown-header">Nav header</li>
                <li><a href="#">Separated link</a></li>
                <li><a href="#">One more separated link</a></li>
              </ul>
            </li>
          </ul>
          <ul class="nav navbar-nav navbar-right">
            <li><a href="../navbar/">Default</a></li>
            <li><a href="../navbar-static-top/">Static top</a></li>
            <li class="active"><a href="./">Fixed top <span class="sr-only">(current)</span></a></li>
          </ul>
        </div><!--/.nav-collapse -->
      </div>
    </nav>

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css
">

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>