Do not do the online copy. It's a total rip off. Once you finish the test, you'll be able to see what you got wrong. HOWEVER, you won't be able to see the correct answers. I called ADA and Prometric and they said they don't provide answers for the online version as of now. (Pretty stupid if you ask me)
On the other hand, if you get the hard copy (which I think is a little more expensive), an answer sheet will be provided. The only thing downside is that you won't be able to practice taking the test on a computer screen.