我已经按照引导程序网站的指示将引导 css js 和 jQuery 添加到页面底部，但每当我添加折叠和导航栏折叠类时，所有的 ul 和 li 都会消失"一个小音符，智能感知不会给我崩溃，它给了我崩溃"。
<!DOCTYPE html>
<html>
<head>
<title>Camp</title>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
</head>
<body>
<nav class="navbar navbar-light bg-light">
<div class="container-fluid">
<div class="navbar-header">
<a class="navbar-brand" href="/">Camp</a>
</div>
<div class="collapse navbar-collapse">
<ul class="nav navbar-nav nav-pills justify-content-end ">
<li class="nav-item"><a href="/">Login</a></li>
<br />
<li class="nav-item"><a href="/">Sign Up</a></li>
<br />
<li class="nav-item"><a href="/">Logout</a></li>
</ul>
</div>
</div>
</nav>
<!DOCTYPE html>
<html>
<head>
<title>Camp</title>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
</head>
<body>
<nav class="navbar navbar-expand-md navbar-light bg-light mb-4" >
<a class="navbar-brand" href="/">Camp</a>
<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 justify-content-end" id="navbarCollapse">
<ul class="navbar-nav mr-auto">
<li class="nav-item"><a class="nav-link" href="/campgrounds">Home</a></li>
</ul>
<ul class="navbar-nav">
<li class="nav-item"><a class="nav-link" href="/login">Login</a></li>
<li class="nav-item"><a class="nav-link" href="/register">Sign Up</a></li>
</ul>
</div>
</nav>