填写缺失的空白处，创建一个导航栏，在小屏幕上应该变为垂直方向。
<nav class="@(6) @(16) bg-light"> <ul class="navbar-nav"> <li class="nav-item"> <a class="nav-link" href="#">链接 1</a> </li> <li class="nav-item"> <a class="nav-link" href="#">链接 2</a> </li> </ul> </nav>
<nav class="navbar navbar-expand-sm bg-light"> <ul class="navbar-nav"> <li class="nav-item"> <a class="nav-link" href="#">链接 1</a> </li> <li class="nav-item"> <a class="nav-link" href="#">链接 2</a> </li> </ul> </nav>