Because it's blatantly obvious that it's a set question. We don't just give out answers to interview questions, it doesn't help you learn and I don't need the job.

Seriously, create a table, create three SSMS sessions and do what's in that list and see how many rows. You'll remember the answer (and possibly the reason) far better than if someone just gives you the answer.

Also, read up on transactions and rollbacks for the theory behind why it works