Which process is a “build up” process

Mike wants to negatively charge a small rubber ball. which of these methods would successfully charge the ball? a) heating the ball in boiling water b) running the ball over a strong magnet c) rubbing the ball back and forth on carpet d) dropping the ball from a tall bulding submit hint forces in nature (s8p5.b) conductors and insulators id: 10402 hint electrons must be transferred.