6 在系统中定义并获取通知服务的代码使哪一个（）？ A: NotificationManager manager = (NotificationManager) getService(NOTIFICATION_SERVICE); B: NotificationManager manager = (NotificationManager) getService(NOTIFICATION_SERVICE_Manager); C: NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE_Manager); D: NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);