导航栏下拉列表无法正常工作

我正在使用mvc。我尝试创建导航栏下拉菜单。我使用以下html代码创建导航栏下拉菜单

 <li class="dropdown">
  <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="menu">
      Report
  <b class="caret"></b>
  </a>
    <ul class="dropdown-menu">
      <li><a href="#">a </a></li>
      <li><a href="#">b</a></li>
      <li><a href="#">c</a></li>
      <li><a href="#">d</a></li>
      <li><a href="#">e</a></li>

    </ul>
  </li>

但是我在UI中得到以下输出。我不知道这个问题

我不知道为什么a和b值没有出现在UI中。有人可以帮我吗？

这是我正在使用的css文件

element.style {
}
.navbar-inverse .navbar-collapse, .navbar-inverse .navbar-form {
    border-color: #101010;
}
@media (min-width: 768px)
.container > .navbar-header, .container-fluid > .navbar-header, .container > .navbar-collapse, .container-fluid > .navbar-collapse {
    margin-right: 0;
    margin-left: 0;
}
.container > .navbar-header, .container-fluid > .navbar-header, .container > .navbar-collapse, .container-fluid > .navbar-collapse {
    margin-right: -15px;
    margin-left: -15px;
}
.navbar-fixed-top .navbar-collapse, .navbar-fixed-bottom .navbar-collapse {
    max-height: 340px;
}
@media (min-width: 768px)
.navbar-fixed-top .navbar-collapse, .navbar-static-top .navbar-collapse, .navbar-fixed-bottom .navbar-collapse {
    padding-right: 0;
    padding-left: 0;
}
@media (min-width: 768px)
.navbar-collapse.collapse {
    display: block !important;
    height: auto !important;
    padding-bottom: 0;
    overflow: visible !important;
    visibility: visible !important;
}
@media (min-width: 768px)
.navbar-collapse {
    width: auto;
    border-top: 0;
    -webkit-box-shadow: none;
    box-shadow: none;
}
.navbar-collapse {
    padding-right: 15px;
    padding-left: 15px;
    overflow-x: visible;
    -webkit-overflow-scrolling: touch;
    border-top: 1px solid transparent;
    -webkit-box-shadow: inset 0 1px 0 rgba(255, 255, 255, .1);
    box-shadow: inset 0 1px 0 rgba(255, 255, 255, .1);
}
.collapse {
    display: none;
    visibility: hidden;
}
* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
user agent stylesheetdiv {
    display: block;
}
Inherited from 
body {
    font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
    font-size: 14px;
    line-height: 1.42857143;
    color: #333;
    background-color: #fff;
}
Inherited from 
html {
    font-size: 10px;
    -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
}
html {
    font-family: sans-serif;
    -webkit-text-size-adjust: 100%;
    -ms-text-size-adjust: 100%;
}
Pseudo ::before element
.clearfix:before, .clearfix:after, .dl-horizontal dd:before, .dl-horizontal dd:after, .container:before, .container:after, .container-fluid:before, .container-fluid:after, .row:before, .row:after, .form-horizontal .form-group:before, .form-horizontal .form-group:after, .btn-toolbar:before, .btn-toolbar:after, .btn-group-vertical > .btn-group:before, .btn-group-vertical > .btn-group:after, .nav:before, .nav:after, .navbar:before, .navbar:after, .navbar-header:before, .navbar-header:after, .navbar-collapse:before, .navbar-collapse:after, .pager:before, .pager:after, .panel-body:before, .panel-body:after, .modal-footer:before, .modal-footer:after {
    display: table;
    content: " ";
}
*:before, *:after {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
Pseudo ::after element
.clearfix:after, .dl-horizontal dd:after, .container:after, .container-fluid:after, .row:after, .form-horizontal .form-group:after, .btn-toolbar:after, .btn-group-vertical > .btn-group:after, .nav:after, .navbar:after, .navbar-header:after, .navbar-collapse:after, .pager:after, .panel-body:after, .modal-footer:after {
    clear: both;
}
.clearfix:before, .clearfix:after, .dl-horizontal dd:before, .dl-horizontal dd:after, .container:before, .container:after, .container-fluid:before, .container-fluid:after, .row:before, .row:after, .form-horizontal .form-group:before, .form-horizontal .form-group:after, .btn-toolbar:before, .btn-toolbar:after, .btn-group-vertical > .btn-group:before, .btn-group-vertical > .btn-group:after, .nav:before, .nav:after, .navbar:before, .navbar:after, .navbar-header:before, .navbar-header:after, .navbar-collapse:before, .navbar-collapse:after, .pager:before, .pager:after, .panel-body:before, .panel-body:after, .modal-footer:before, .modal-footer:after {
    display: table;
    content: " ";
}
*:before, *:after {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}