A wave y = 10 sin (ax + bt) is reflected from a dense medium at an origin. If 81% of energy is reflected then the equation of reflected wave is:
(a) y = -8.1 sin (ax - bt)
(b) y = 8.1 sin (ax+bt)
(c) y = -9 sin (bt - ax)
(d) y = 10 sin (ax - bt) port