justify-content-md-center 不起作用？

Question

我正在学习教程，我想在达到中等屏幕尺寸时使用 justify-content-md-center。但它似乎不起作用。

 <div class="container">
    <div class="row" style="background:lightblue">
      <div class="col-md-2">col-sm-6</div>
      <div class="col-md-8">col-sm-6</div>
      <div class="col-md-2">col-sm-6</div>
    </div>
      <div class="row justify-content-md-center">
        <div class="col-md-2">col-sm-2</div>
        <div class="col-md-2">col-sm-2</div>
        <div class="col-md-2">col-sm-2</div>
      </div>

  </div>

我感谢帮助谢谢

Answer 1

希望这有效

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet" />
   

<div class="row justify-content-center" style="background: darkgray;">
  <div class="col-xs-2">
    <img src="https://placehold.it/150x150" alt="sample image">
  </div>
  
  <div class="col-xs-2">
    <img src="https://placehold.it/150x150" alt="sample image">
</div>
</div>