Bootstrap下拉头部

<div class = "dropdown">

   <button type = "button" class = "btn dropdown-toggle" id = "dropdownMenu1" data-toggle = "dropdown">
      Topics
      <span class = "caret"></span>
   </button>

   <ul class = "dropdown-menu" role = "menu" aria-labelledby = "dropdownMenu1">
      <li role = "presentation" class = "dropdown-header">Dropdown header</li>

      <li role = "presentation" >
         <a role = "menuitem" tabindex = "-1" href = "#">Java</a>
      </li>

      <li role = "presentation">
         <a role = "menuitem" tabindex = "-1" href = "#">Data Mining</a>
      </li>

      <li role = "presentation">
         <a role = "menuitem" tabindex = "-1" href = "#">
            Data Communication/Networking
         </a>
      </li>

      <li role = "presentation" class = "divider"></li>
      <li role = "presentation" class = "dropdown-header">Dropdown header</li>

      <li role = "presentation">
         <a role = "menuitem" tabindex = "-1" href = "#">Separated link</a>
      </li>
   </ul>

</div>