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 the other, their amplitudes adds up