There are three possible responses to this question:
1. You should keep the door you originally chose because the probability of winning
if you keep it is greater than the probability of winning if you switch to the other
unopened door.
2. You should switch to the other unopened door because the probability of winning
if you switch is greater than the probability of winning if you keep the door you
originally chose.
3. It doesn't matter whether you keep the original door or switch to the other
unopened door because the probability of winning is the same either way.