要想获取select的值，使用ng-modle，否则无法获取select 的值

ng-bind是从$scope -> view的单向绑定
ng-modle是$scope <-> view的双向绑定

                    <form role="form" class="form-inline">
                    
                        <div class="form-group">
                            <label for="nj">年级:</label>
                            <select name="nj" >>
                                <option ng-repeat="op in njs " value="{{op.id}}">{{op.name}}</option>
                            </select>
                       </div>
                       
                        <div class="form-group">
                            <label for="zy">年级:</label>
                            <select name="zy" >
                                <option ng-repeat="op in zys " value="{{op.id}}">{{op.name}}</option>
                            </select>
                        </div>
                        
                       <button type="submit"  class="btn btn-primary"  >查询</button>
                       
                    </form>