PrimeNG列表框一次选择全部而不是一个

<p-listbox [options]="cities" [(ngModel)]="selectedCities" [multiple]="true" [checkbox]="true">
  <ng-template let-city pTemplate="item">
    <div class="ui-chkbox ui-widget">
      <div class="ui-chkbox-box ui-widget ui-corner-all ui-state-default">
        <span class="ui-chkbox-icon ui-clickable" [ngClass]="{'pi pi-check': city.selected}"></span>
      </div>
    </div>
    <span>{{city.label}}</span>
  </ng-template>
</p-listbox>

import { Component } from '@angular/core';
import { SelectItem } from 'primeng/api';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  cities: SelectItem[];
  selectedCities: string[];

  constructor() {
    this.cities = [
      { label: '全部', value: null },
      { label: 'New York', value: 'New York' },
      { label: 'San Francisco', value: 'San Francisco' },
      { label: 'Los Angeles', value: 'Los Angeles' }
    ];
    this.selectedCities = [];
  }
}