dropdown下拉列表是什么作用？怎么不能选中菜单啊？

<div class="dropdown">

  <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu2" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">

    Dropup

    <span class="caret"></span>

  </button>

  <ul class="dropdown-menu" aria-labelledby="dropdownMenu2">

    <li  class="dropdown-header"><a href="#">苹果</a></li>

    <li><a href="#">橘子/a></li>

    <li role="separator" class="divider">香蕉</li>

    <li><a href="#">Separated link</a></li>

  </ul>

</div>