While skateboarding at 19 km/h, alana throws a tennis ball at 11 km/h to her friend oliver. if alana is the reference frame, the speed of the tennis ball is km/h.

Which statement best describes the superposition principle? a.) if two in-phase waves arrive simultaneously at a point, their amplitudes add up b.) if two out-of-phase waves arrive simultaneously at a point, their amplitudes add up c.) if two in-phase waves arrive at a point one after another, their amplitudes add up d.) if two out-of-phase waves arrive at a point one after another, their amplitudes adds up