| 导航栏上可以设置下拉菜单: 
 实例:
 
 <!DOCTYPE html>
 <html>
 <head>
 <title>Bootstrap4 实例</title>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank"  rel="external nofollow" target="_blank" >
 <script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
 <script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
 <script src="https://cdn.bootcss.com/bootstrap/4.1.0/js/bootstrap.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
 </head>
 <body>
 <nav class="navbar navbar-expand-sm bg-dark navbar-dark">
 <!-- Brand -->
 <a class="navbar-brand" href="#">Logo</a>
 <!-- Links -->
 <ul class="navbar-nav">
 <li class="nav-item">
 <a class="nav-link" href="#">Link 1</a>
 </li>
 <li class="nav-item">
 <a class="nav-link" href="#">Link 2</a>
 </li>
 <!-- Dropdown -->
 <li class="nav-item dropdown">
 <a class="nav-link dropdown-toggle" href="#" id="navbardrop" data-toggle="dropdown">
 Dropdown link
 </a>
 <div class="dropdown-menu">
 <a class="dropdown-item" href="#">Link 1</a>
 <a class="dropdown-item" href="#">Link 2</a>
 <a class="dropdown-item" href="#">Link 3</a>
 </div>
 </li>
 </ul>
 </nav>
 </body>
 </html>
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