Select with default value default Angularjs

Just by defining an ng-Model select does not default to all values. Does anyone know why? I start StatusRespNaoResp = 0 in the controller.

<div class="btn-group">
    <select name="iconeTopico" id="iconeResp" class="spam3" style="font-family: 'FontAwesome', Helvetica;"  ng-model="StatusRespNaoResp" ng-dropdown required ng-change="CarregaListaPerguntaPorFiltro(StatusRespNaoResp)">
        <option ng-option value=0 selected disabled hidden>Todas</option>
        <option ng-option value=1>Respondidas</option>
        <option ng-option value=2>Não Respondidas</option>
    </select>
</div>