OK....let's walk this through. You have four choices: A, B, C, and D. One of them is guaranteed ahead of time. For the sake of example let's say that it's "C". When the "decision" comes, can you choose "A"?
You make the same mistake as JMorris, in my opinion. Yes, the choice is guaranteed ahead of time, but it has been guaranteed ahead of time by your own choice. Why should we get the chance to choose twice?
Note: Though I believe this to be a good argument, my preferred argument is the one displayed in posts 554 and 570.