Which expression will complete the proof?
a. cos(k0 - 0) + i sin(k0 - 0)
b. cos(k0 - 0) + i sin(k0+ 0)
c. cos(k0 + 0) + i sin(k0 - 0)
d. cos(k0 + 0) + i sin(k0 + 0)
