In a free-body diagram for a block sliding to the right on a horizontal surface, how does kinetic friction affect the block's motion when no other horizontal forces act on it?

• A. It acts upward and reduces the normal force, changing only the vertical motion.
• B. It has no effect unless the surface is inclined, so the block keeps moving at constant speed.
• C. It acts opposite the direction of motion and causes the block to slow down (acceleration to the left).
• D. It acts in the direction of motion and causes the block to speed up (acceleration to the right).

Answer: (C)