试着在bootstrap中做一个navbar。标题占用了所有空间，折叠div没有显示。而我从教程中输入了完全生成输出的精确代码。我做错了什么？这里是Bootstrap初学者。
我创建了一个带有navbar-hear和navbar-collable div的nav标记，其中有两个navbar选项列表
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<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<nav class="nav navbar-default">
<div class="container-fluid">
<div class="navbar-header">
<a href="#" class="navbar-brand">Recipe Book</a>
</div>
<div class="navbar-collapse collapse">
<ul class="navbar-nav nav">
<li><a href="#">Recipes</a></li>
<li><a href="#">Shopping</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li class="dropdown">
<ul class="dropdown-menu">
<a href="#" class="dropdown-toggle">Manage <span class="caret"></span></a>
<li><a href="#">Save Data</a></li>
<li><a href="#">Fetch Data</a></li>
</ul>
</li>
</ul>
</div>
</div>
</nav>null
navbar-nav元素是不可见的，如果我移除caplable class，它们只是出现在navbar-header下面，这占用了太多空间。
这就是你想要做的吗？在不需要的地方使用了一些类。
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a href="#" class="navbar-brand">Recipe Book</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav nav">
<li><a href="#">Recipes</a></li>
<li><a href="#">Shopping</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Manage
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Save Data</a>
<a class="dropdown-item" href="#">Fetch Data</a>
</div>
</li>
</ul>
</div>
</nav>