Jack knew about the voting off by their peers the night before they were to do it. They were instructed they couldn't tell him of the twist. However it should be Jack's choice who he wants to let go each week, not the girls. You didn't see the Bachelor or Bachelorette have the girls/guys to vote off one of their own. That was a stupid move, but its part of the twist of the return of Marissa. The other girls are not being compeitive, instead they are whining because Marissa is so aggressive. So instead of wasting time whining, why don't they step up to the bat, stop being whiners and compete for his affections. After all thats the name of the game.
