On friction between two given surfaces:

a) the kinetic frictional force is always in the direction opposite to the direction of motion

b) if you slightly push one surface relative to another without actually moving it, the resultant static friction force is always equal to the coefficient of static friction times the normal force.

c) the coefficient of kinetic friction depends on the smoothness of the surfaces

d) the magnitude of the frictional force cannot be bigger than the magnitude of the normal force.