21 選出正確的敘述(A)波從輕弦入射重弦，則反射波與入射波反向(out of phase)(B)波從重弦入射輕弦，則反射波與入射波同向(in phase)(C)波的振幅加倍，則傳遞的功率增大四倍(D)弦的線密度減半，則傳遞的功率也減半(E)弦的張力加倍，則傳遞的功率也加倍