Concept:Friction is a useful force that can be used to do work such as shaping or sharpening objects.Explanation:Friction is the resistance between two surfaces in contact.When a cutting tool is rubbed against a rough surface, friction wears away tiny particles of the metal, forming a sharp edge.This is why grinding and sharpening tools depend on friction.The other options describe disadvantages of friction: reducing machine efficiency, producing unwanted heat, and wearing out shoe soles.Answer:A. enables cutting tools to be sharpened.