响应式布局

<div class="row">
<div class="box1 col-xs-12 col-md-4">A</div>
<div class="box2 col-xs-12 col-md-8 col-sm-3">B</div>
<div class="col-xs-12 col-sm-3">C</div>
<div class="col-xs-12 col-sm-3">D</div>
</div>