angular2 -如何设置选择框的初始值

<select [(ngModel)]="selectedValue">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>

export class MyComponent {
  selectedValue: string = "option2";
}

<select [(ngModel)]="selectedValue">
  <option [ngValue]="option1">Option 1</option>
  <option [ngValue]="option2">Option 2</option>
  <option [ngValue]="option3">Option 3</option>
</select>

export class MyComponent {
  selectedValue: string = "option2";
  option1: string = "option1";
  option2: string = "option2";
  option3: string = "option3";
}

<select [(ngModel)]="selectedValue">
  <option [ngValue]="'option1'">Option 1</option>
  <option [ngValue]="'option2'">Option 2</option>
  <option [ngValue]="'option3'">Option 3</option>
</select>

export class MyComponent {
  selectedValue: string = "option2";
}