我有一个引导表单。当使用键盘Tab键时，从第一个字段开始，我可以在表单字段中导航。但是，我有一个单选按钮条目（管理员/操作员），按Tab键会跳过它。
如何通过按Tab键来访问表单的单选按钮？

<form class='form-horizontal'>
  <div class='control-group'>
    <label class='control-label'>Username</label>
    <div class='controls'>
      <input type='text' class='username' id='new-user-username' placeholder='Username'
      />
    </div>
  </div>
  <div class='control-group'>
    <label class='control-label'>Rights</label>
    <div class='controls'>
      <div class='btn-group' data-toggle='buttons-radio' id='new-user-rights'> <span class='btn'>Administrator</span>
 <span class='btn' id='new-user-operator'>Operator</span>

      </div>
    </div>
  </div>
  <div class='control-group'>
    <label class='control-label'>Password</label>
    <div class='controls'>
      <input type='password' class='password' id='new-user-password' placeholder='Password'
      />
    </div>
  </div>
  <div class='control-group'>
    <label class='control-label'>Confirm password</label>
    <div class='controls'>
      <input type='password' class='confirm' id='new-user-confirm' placeholder='Confirm password'
      />
    </div>
  </div>
  <div class='controls'>
    <button type='button' class='btn btn-primary' id='new-user-button'>Add new user</button>
  </div>
</form>