A block slides down a rough inclined plane that makes an angle of 30∘ with the horizontal. Three forces act on the block:
(I) Weight of the block (vertically downward)
(II) Normal reaction exerted by the plane on the block (perpendicular to the plane)
(III) Kinetic friction acting up the plane (along the surface)
Which one of these forces does zero work on the block while it slides down the plane?
a) Only force I
b) Only force II
c) Only force III
d) Forces II and III