A traveling wave passes from a heavier string to a lighter string. The reflected wave has amplitude 0.5a. The junction of the strings is at x = 0. The equation of the reflected wave is:

a) 0.5a cos(kx - ωt)
b) 0.5a sin(kx + ωt)
c) 0.5a cos(kx + ωt)
d) 0.5a sin(kx - ωt)