<ul class="nav nav-pills nav-stacked text-center" style="padding-right:2px;" >
  <li>
    <a href="#collapse1" id="btn-1" class="text-center" data-toggle="collapse"><span><b>A</b></span> </a>
    <ul class="nav nav-pills nav-stacked text-center collapse " id="collapse1" role="menu" aria-labelledby="btn-1" aria-expanded="false">
      <li ><a href="~/A/A-a">A-a</a></li>
      <li ><a href="~/A/A-b">A-b</a></li>
      <li ><a href="~/A/A-c">A-c</a></li>
    </ul>
  </li>

  <li>
    <a href="#collapse2" class="text-center" data-toggle="collapse"><span><b>B</b></span></a>
    <ul id="collapse2" class="collapse nav nav-pills nav-stacked text-center collapse" aria-expanded="false">
      <li ><a href="~/B/B-a">B-a</a></li>
      <li ><a href="~/B/B-b">B-b</a></li>
    </ul>
  </li>
</ul>

<li ><a href="~/B/B-b">B-b</a></li>