用Bootstrap 4创建一个块元素
要创建一个块元素，请使用.d-blockBootstrap4类。
使用d-block类-
<span class="d-block bg-info"> A block </span>
要设置不同屏幕尺寸的块，请使用.d-*-block-
<span class="d-md-block bg-info"> A block for medium screen size </span>
让我们看一个例子来学习如何创建块元素-
示例
<!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/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
<h2>Blocks</h2>
<p>Resize the browser to check the effect</p>
<span class="d-block bg-info">
A block
</span>
<span class="d-md-block bg-info">
A block for medium screen size
</span>
</div>
</body>
</html>