angularjs的ng-options时如何设置默认值(初始值)

hhtml:

<select ng-change="change2()" ng-model="selected"
        ng-options="item.id for item in datas">
</select>

angualrJs:

$http.get('url').then(function (res) {
  $scope.datas= res.data;

$scope.selected= $scope.datas[0];

});