title: Select drop down does not work in mobile device when apply bootstrap
body: <p>I'm using html select in the bootstrap <strong>col</strong>, but when i click drop down button in the mobile device(iOS and Android), it can not display the option list. But it works in the PC browser. The html code is like below:</p>
  
  <pre><code>&lt;div class="row"&gt;
      &lt;div class="col-xs-6 col-sm-6 col-md-6"&gt;
          &lt;select&gt;
              &lt;option value="1"&gt;1&lt;/option&gt;
              &lt;option value="2"&gt;2&lt;/option&gt;                
              &lt;option value="3"&gt;3&lt;/option&gt;   
          &lt;/select&gt;
       &lt;/div&gt;
       &lt;div class="col-xs-6 col-sm-6 col-md-6"&gt;
          &lt;select&gt;
              &lt;option value="a"&gt;a&lt;/option&gt;
              &lt;option value="b"&gt;b&lt;/option&gt;                
              &lt;option value="c"&gt;c&lt;/option&gt;   
          &lt;/select&gt;
       &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>If i remove those "col-xs-6 col-sm-6 col-md-6" class from div, the select drop down works in the mobile device.</p>
  
  <p>So what's the issue? How can i make it work?</p>