In the context of free-body diagrams, which statement best describes friction?

• A. A contact force that acts parallel to the surfaces in contact and opposes their relative motion or tendency to slip
• B. A contact force that acts perpendicular to the surface and prevents interpenetration (the normal force)
• C. A force that always points in the direction of an object's velocity and increases its speed
• D. A long-range force that attracts masses with magnitude $F=G\frac{m_1{m}_2}{r^2}$

Answer: (A)