下拉菜单

//核心代码 dropdown  data-toggle="dropdown" class="dropdown-menu"
<div class="dropdown">
    <button class="btn btn-default " type="button"  data-toggle="dropdown" >
        Dropdown
        <span class="caret"></span>
    </button>
    <ul class="dropdown-menu">
        <li><a href="#">Action</a></li>
        <li><a href="#">Another action</a></li>
        <li><a href="#">Something else here</a></li>
        <li><a href="#">Separated link</a></li>
    </ul>
</div>