我正在尝试学习 flexbox。我想用 Bootstrap 设计响应式卡片。在图中显示的 4 张卡片中，2 张应该出现在底部，另外两张出现在顶部，但在手机上它们应该都在底部。我尝试了一些东西，但没有奏效。我该怎么办？
谢谢。
.card{
display: flex;
flex-wrap: wrap;
}
@media (max-width: 500px) {
.card{
flex-direction: row;
}
}<!DOCTYPE html>
<html>
<head>
<title>A</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/css/bootstrap.min.css" integrity="sha384-B0vP5xmATw1+K9KRQjQERJvTumQW0nPEzvF6L/Z6nronJ3oUOFUFpCjEUQouq2+l" crossorigin="anonymous">
<link rel="stylesheet" type="text/css" href="style.css">
<meta name="viewport" content="width=device-width,initial-scale=1.0">
</head>
<body>
<div class="container" style="margin-top: 50px;">
<div class="card-deck">
<div class="card mb-4">
<img class="card-img-top img-fluid" src="https://via.placeholder.com/500x280" alt="Card image cap">
<div class="card-body">
<h4 class="card-title">1 Card title</h4>
<p class="card-text">This is a longer card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
<div class="card mb-4">
<img class="card-img-top img-fluid" src="https://via.placeholder.com/500x280" alt="Card image cap">
<div class="card-body">
<h4 class="card-title">2 Card title</h4>
<p class="card-text">This is a longer card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
<div class="w-100 d-none d-sm-block d-md-none"><!-- wrap every 2 on sm--></div>
<div class="card mb-4">
<img class="card-img-top img-fluid" src="https://via.placeholder.com/500x280" alt="Card image cap">
<div class="card-body">
<h4 class="card-title">3 Card title</h4>
<p class="card-text">This is a longer card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
<div class="w-100 d-none d-md-block d-lg-none"><!-- wrap every 3 on md--></div>
<div class="card mb-4">
<img class="card-img-top img-fluid" src="https://via.placeholder.com/500x280" alt="Card image cap">
<div class="card-body">
<h4 class="card-title">4 Card title</h4>
<p class="card-text">This is a longer card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
</div>
</div>
<script src="https://code.jquery.com/jquery-3.5.1.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/js/bootstrap.bundle.min.js" integrity="sha384-Piv4xVNRyMGpqkS2by6br4gNJ7DXjqk09RmUpJ8jgGtD7zP9yug3goQfGII0yAns" crossorigin="anonymous"></script>
<script src="https://code.jquery.com/jquery-3.5.1.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.1/dist/umd/popper.min.js" integrity="sha384-9/reFTGAW83EW2RDu2S0VKaIzap3H66lZH81PoYlFhbGU+6BZp6G7niu735Sk7lN" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/js/bootstrap.min.js" integrity="sha384-+YQ4JLhjyBLPDQt//I+STsc9iw4uqqACwlvpslubQzn4u2UU2UFM80nGisd026JF" crossorigin="anonymous"></script>
</body>
</html>
