<!doctype html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<meta name="robots" content="noindex, follow">
<!-- Bootstrap CSS -->
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.1/dist/css/bootstrap.min.css" rel="stylesheet">
<title>Hello, world!</title>
<style>
.row * {
border: 4px solid black
}
</style>
</head>
<body>
<br>
<div class="container">
<div class="row">
<div class="col-6 col-md-3">1</div>
<div class="col-6 col-md-3">2</div>
<div class="col-6 col-md-3">3</div>
<div class="col-6 col-md-3">4</div>
</div>
<div class="row">
<div class="col-12 col-md-6 col-lg">1</div>
<div class="col-12 col-md-6 col-lg">2</div>
<div class="col-12 col-md-6 col-lg">3</div>
<div class="col-12 col-md-6 col-lg">4</div>
<div class="col-12 col-md-6 col-lg">5</div>
<div class="col-12 col-md-6 col-lg">6</div>
<div class="col-12 col-md-6 col-lg">7</div>
<div class="col-12 col-md-6 col-lg">8</div>
<div class="col-12 col-md-6 col-lg">9</div>
<div class="col-12 col-md-6 col-lg">10</div>
</div>
</div>
</body>
</html>