col里嵌套row,被嵌套的那部分栅格布局没有效果了,为什么啊?
<div class="container">
<div class="row">
<div class="col-sm-9">
<div class="jumbotron">
<h1>Hello, world!</h1>
<p>基本介绍</p>
</div>
<div class="row">
<div class="col-6">
<h2> 标题 </h2>
<p> fsdhfhsjkhskfhksaffsfjlsfjkdsjfklsdjflskfjlksjflksjflsjflsjdflkdsavhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhkjsahldfjkfdassssssssssssssssssssssssssssssssss </p>
<a class="btn btn-default"> 更多</a>
</div>
</div>
</div>
<div class="col-sm-3">
<ul class="list-group">
<li class="list-group-item">Cras justo odio</li>
<li class="list-group-item">Dapibus ac facilisis in</li>
<li class="list-group-item">Morbi leo risus</li>
<li class="list-group-item">Porta ac consectetur ac</li>
<li class="list-group-item">Vestibulum at eros</li>
</ul>
</div>
</div>
</div>