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玩转bootstrap中间的间隔一直出不来？

<div class="mid">
      <ul>
        <li class="li1"><img src="http://img1.sycdn.imooc.com//53edad54000119fb03300130.jpg"/><br/>
            <a href="#">玩转bootstrap</a></li>
        <li class="li2"><img src="http://img1.sycdn.imooc.com//53edad690001260a03300130.jpg"/><br/>
            <a href="#">企业网页实战</a></li>
        <li class="li3"><img src="http://img1.sycdn.imooc.com//53edad7a0001cde803300130.jpg"/><br/>
            <a href="#">JavaScript</a></li>
      </ul>
    </div>

.mid{
   width:1000px;
   height:150px;
   margin:10px auto;}
.mid li{
   list-style-type:none;
   text-align:center;
   float:left;
    width: 330px;
    height: 130px;}
.mid li1,.mid li2{
   margin-right:5px;}
.mid li3{
   margin-right:0;}
.mid a{
   text-decoration:none;
   color:#000;}