如图一，只能用另一个div标签来换行，在同一个div标签里，即使用了br标签和hr标签，既没有实现“按钮3”的换行，也没有分割线：
图二中，黄色箭头所示，两行代码为何都没有生效？
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>按钮组</title>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css" integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu" crossorigin="anonymous">
</head>
<body>
<div class="btn-group" role="group">
<button type="button" class="btn btn-primary">按钮1</button>
<button type="button" class="btn btn-secondary">按钮2</button>
<br/>
<hr/>
<button type="button" class="btn btn-outline-success">按钮3</button>
</div>
<hr/>
<div>
<button type="submit" class="btn btn-primary">Button Submit</button>
<input type="button" class="btn btn-primary" value="input button" />
</div>
</body>
</html>
