An office worker has his lunch on the 53rd floor of an office building. In which case is the gravitational potential energy of the lunch-Earth system greatest: \((a)\) the worker carried the lunch up to the 53rd floor in the elevator, \((b)\) he carried it up the stairs to the 53rd floor, or (c) he got a ride in a helicopter to the top of the 68-floor building and then carried the lunch down in the elevator to the 53rd floor?