There are three controllers here, `AViewController`, `BViewController`, `CViewController`,
the first step: `AViewController` present to `BViewController`;

```
BViewController *BVC = [[BViewController alloc]init];
[self presentViewController:BVC animated:YES completion:nil];

```

The second step: `BViewController` push to `CViewController`;

```
CViewController *CVC = [[CViewController alloc]init];
UINavigationController *nav = [[UINavigationController alloc]initWithRootViewController:CVC];
[self.navigationController pushViewController:nav animated:YES];

```

Now, if I want to go back from `CViewController` to `AViewController`, what code should I write?