我正在尝试使用Bootstrap创建一个响应式导航栏。单击超小和小屏幕尺寸上的切换器图标时，看不到折叠的内容。有什么问题？以下是我的代码。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=n">
<link rel="stylesheet" href="node_modules/bootstrap/dist/css/bootstrap.min.css">
<link rel="stylesheet" href="node_modules/font-awesome/css/font-awesome.min.css">
<link rel="stylesheet" href="node_modules/bootstrap-social/bootstrap-social.css">
<link rel="stylesheet" type="text/css" href="./style.css">
<title>MONGO DB</title>
</head>
<body>
<nav class="navbar navbar-expand-md navbar-light bg-light fixed-top">
<div class="container-fluid">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#defaultNavbar1">
<span class="navbar-toggler-icon"></span>
</button>
<a class="navbar-brand mr-auto" href="#">mongoDB</a>
<div class="collapse navbar-collapse" id="defaultNavbar1">
<ul class="navbar-nav">
<li class="nav-item active"><a class="nav-link" href="#">Cloud</a></li>
</ul>
<ul class="navbar-nav">
<li class="nav-item"><a class="nav-link" href="#">Software</a></li>
</ul>
<ul class="navbar-nav">
<li class="nav-item"><a class="nav-link" href="#">Learn</a></li>
</ul>
<ul class="navbar-nav fourth">
<li class="nav-item"><a class="nav-link" href="#">Solutions</a></li>
</ul>
<ul class="navbar-nav">
<li class="nav-item"><a class="nav-link" href="#">Docs</a></li>
</ul>
</div>
</div>
</nav>
<script src="node_modules/jquery/dist/jquery.slim.min.js"></script>
<script src="node_modules/popper.js/dist/umd/popper.min.js"></script>
<script src="node_modules/bootstrap/dist/js/bootstrap.min.js"></script>
</body>
</html>