angular select 默认值如何设置

如下所示：

<select ng-model="selected" ng-options="x.id as x.name for x in users"></select>

$scope.users = [    {name:'a',id:'1'},    {name:'b',id:'2'},    {name:'c',id:'3'}  ];  $scope.selected='2';//id的值，区分类型  $scope.selected=$scope.users[0].id;//如果想要第一个值