Homework help regarding forces of friction and their directions

[MechRocket]

The Problem

http://img15.imgspot.com/u/07/118/16/friction.JPG

What I Think

http://img15.imgspot.com/u/07/118/16/think.JPG

The Correct Solution

http://img15.imgspot.com/u/07/118/16/solution.JPG

What Is Confusing Me

Shouldn't the force of friction on the slab be pointed to the left? I knew the force of friction on the block is pointed to the left (because friction will oppose movement), so I naturally assumed this force to the left also acts on the slab (the two surfaces are touching after all).

I just cannot see why the friction force on the slab is to the right.

Thanks a lot! :)

 

[Saladsamurai]

Picture instead of the slab remaining stationary and the block sliding right, the block standing still and i the slab is sliding left. . . then which direction will oppose motion direction?