为什么工具提示不适用于select元素的选项标签

Question

当有人将鼠标悬停在select标签的选项上时，我正在使用uib-tooltip-html显示工具提示。但它不起作用。

＆＃13;

＆＃13;

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" />

<select id="repeatSelect" ng-model="$root.selectedClusterOption" class="cluster-options btn btn-default dropdown-toggle" ng-change="header.goToClusterPage()">
  <option value="allClusters" ng-selected="$root.selectedClusterOption === 'allClusters'" uib-tooltip="All Clusters" tooltip-placement="auto bottom-left">All Clusters</option>
  <option ng-repeat="cluster in [{cluster_id: 1, cluster_name: 'cluster 1'}]" value="{{cluster.cluster_id}}" ng-selected="$root.selectedClusterOption === '{{cluster.cluster_id}}'" uib-tooltip-html="'{{cluster.cluster_name}}'" tooltip-placement="auto bottom">{{cluster.cluster_name}}</option>
 </select>

＆＃13;

＆＃13;

＆＃13;