如何在angular 4中显示<select2>标签中的占位符？

<select [ngModel]="selectedValue">
  <option value="" disabled selected>请选择</option>
  <option *ngFor="let option of options" [value]="option.value">{{ option.label }}</option>
</select>

selectedValue: string;
options: any[] = [
  { value: 'option1', label: '选项1' },
  { value: 'option2', label: '选项2' },
  { value: 'option3', label: '选项3' }
];

import { Component, OnInit } from '@angular/core';
declare var $: any;

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponentComponent implements OnInit {
  selectedValue: string;
  options: any[] = [
    { value: 'option1', label: '选项1' },
    { value: 'option2', label: '选项2' },
    { value: 'option3', label: '选项3' }
  ];

  ngOnInit() {
    $(document).ready(() => {
      $('select').select2({
        placeholder: '请选择',
        allowClear: true
      });
    });
  }
}