在angular2中选择无法设置默认文本

<select>
  <option value="" disabled selected hidden>请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<select [(ngModel)]="selectedOption">
  <option value="" disabled>请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

selectedOption: string = "请选择";

<select>
  <option *ngIf="!selectedOption" value="" disabled selected hidden>请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

selectedOption: string;

// 初始化为默认文本
ngOnInit() {
  this.selectedOption = null;
}