<header>
<nav class="navbar navbar-expand-md navbar-dark fixed-top bg-dark">
<div class="container">
<a class="navbar-brand" href="#">
<img class="navbar-brand d-sm-none d-md-block" src="img/logo.png"></a>
</div>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarCollapse">
<ul class="navbar-nav ml-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Servicer</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Trailer List</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Gallery</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">News</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Contact</a>
</li>
</ul>
</div>
</nav>
</header>
我想让导航品牌形象只出现在中号及以上的位置，当我缩小到小号时，它又重新出现在汉堡包选项卡上。我一直在重新观看bootstrap 4部分在colt steel的web开发者训练营，并不断错过的东西。
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>
</head>
<body style="height:1500px">
<div class="container">
<nav class="navbar navbar-expand-sm bg-dark navbar-dark sticky-top">
<a class="navbar-brand" href="#">
<img class="navbar-brand d-sm-none d-md-block" src="img/logo.png">
</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#collapsibleNavbar">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="collapsibleNavbar">
<ul class="navbar-nav ml-auto" id="collapsibleNavbar">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Servicer</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Trailer List</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Gallery</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">News</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Contact</a>
</li>
</ul>
</div>
</nav>
</div>
</body>
</html>