GCD应用，如何实现这个需求：A、B、C 三个任务并发，完成后执

dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, globalQueue, ^{
    NSLog(@"001");
});
dispatch_group_async(group, globalQueue, ^{
    NSLog(@"002");
});
dispatch_group_async(group, globalQueue, ^{
    NSLog(@"003");
});
dispatch_group_notify(group, globalQueue, ^{
    NSLog(@"004");
});