填写缺失的空白以创建一个在小屏幕上应垂直的导航栏：
<nav class="@(6) @(16) bg-light">
<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>
</ul>
</nav>
<nav class="navbar navbar-expand-sm bg-light">
<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>
</ul>
</nav>