It's unclear, but a few people have implied that doing nothing isn't making the choice. I disagree with that 100%. When you get to the booth, you're presented with two choices - A or B. It doesn't matter in the slightest that the switch is in the A position. If you leave it there, you've chosen A as much as if when you got there it was in position B and you had to make the switch to A.
Second, I thought this was a trick question. In my mind there is no difficult choice at all. Sacrifice the one to save the many.