Show a select field with options YES/NO without having a default option selected

Question

I'm working on angularjs and I have a field that has two options YES/NO.

When someone will fill this field I need to show the two options but non of them is selected. So it will look like an empty value, but this empty value should not be an option. I tried to add <option value="?"></option> in the following code, but then the empty value is an option. Any ideas?

<div class="col-md-3" ng-if="obj.options.length > 0">
                        <select class="form-control" ng-model="patientReportedSymptomsUI[key]" ng-disabled="$root.newVisit">
                            <option value="?"></option>
                            <option ng-repeat="option in obj.options">{{option}}</option>
                        </select>
                    </div>