AngularJS Multiselect checkbox dropdown with Selected Checkbox

I have a Simple Multiselect checkbox dropdown. As I have given below.

<select name="edit_tags" class="form-control" id="advisor_article_tagsx" multiple="" required>
      <option ng-repeat="service in services" value=""></option>
</select>

Now I want that some checkbox like index 1,3,5 comes with selected checkbox.

My AngularJS Code is given below.

$scope.article_edit = function(articles) {
  $scope.services = [$scope.model[0], $scope.model[1]];
} 

Then What I need to do for that ??

Thanks