In the context of kinetic friction between two solid surfaces, which method will most directly reduce the kinetic frictional force $f_k$ for a given normal force $N$?

• A. Apply a lubricant between the surfaces to reduce the coefficient of kinetic friction ${\mu }_k$.
• B. Increase the normal force $N$ by pressing the surfaces together more strongly.
• C. Increase the contact area while keeping the materials and normal force the same.
• D. Increase the roughness of both surfaces to increase mechanical interlocking.

Answer: (A)