尝试检查Angular中的单选按钮，但不起作用？

Question

所以我只想要一个默认选中的单选按钮（在这种情况下，我希望默认选择“否”）。这是我尝试过的所有方法...

表单组优先

<div class="form-group">
                <div ng-app>
                    <div ng-init="externalClientNo">
                    <label class="control-label" for="approvalrequired">Will be used for External Client:</label>
                    <span class="help-tip">
                        <p>Select Yes or No</p>
                    </span>
                    <div class="form-inline">
                        <div class="radio">
                            <input type="radio" id="externalClientYes" class="form-control" ng-model="externalClientYes" formControlName="approvalrequired" value="Y" (click)="Ext('Y')"/>
                            <label for="externalClientYes">Yes</label>
                        </div>
                        <div class="radio">
                            <input type="radio" id="externalClientNo" ng-model="externalClientNo" class="form-control" formControlName="approvalrequired" value="N" (click)="Ext('N')"/>
                            <label for="externalClientNo">No</label>
                        </div>
                    </div>
                </div>
            </div>
        </div>

NG-APP FIRST

<div ng-app>
        <div ng-init="externalClientNo">
            <div class="form-group">
                    <label class="control-label" for="approvalrequired">Will be used for External Client:</label>
                    <span class="help-tip">
                        <p>Select Yes or No</p>
                    </span>
                    <div class="form-inline">
                        <div class="radio">
                            <input type="radio" id="externalClientYes" class="form-control" ng-model="externalClientYes" formControlName="approvalrequired" value="Y" (click)="Ext('Y')"/>
                            <label for="externalClientYes">Yes</label>
                        </div>
                        <div class="radio">
                            <input type="radio" id="externalClientNo" ng-model="externalClientNo" class="form-control" formControlName="approvalrequired" value="N" (click)="Ext('N')"/>
                            <label for="externalClientNo">No</label>
                        </div>
                    </div>
                </div>
            </div>
        </div> 

已检查

<input type="radio" id="externalClientNo" ng-model="externalClientNo" class="form-control" formControlName="approvalrequired" value="N" (click)="Ext('N') checked"/>
                            <label for="externalClientNo">No</label>

AND

<input type="radio" id="externalClientNo" ng-model="externalClientNo" class="form-control" formControlName="approvalrequired" value="N" (click)="Ext('N')" checked="checked"/>
                            <label for="externalClientNo">No</label>

我明白了为什么选中的方法可能不起作用，但第一种方法应该起作用。我已经看到了演示的工作原理，这就是文档所说的实现方式。我想念什么吗？

谢谢！