I The frictional force is independent of the ar... - JAMB Physics 2000 Question

I. The frictional force is independent of the area of the surfaces in conduct

II. The frictional force depends on the nature of the surfaces in contact

III. The frictional force depends on the speed of sliding

IV. The frictional force is directly proportional to the normal reaction.

Which combination of the above is true of sliding friction?