21. The coefficients of static and sliding friction for wood on wood are respectively 0.5 and 0.3. If a 100 N wooden box is continually pushed across a horizontal wooden floor with just enough force to overcome the force of static friction, the moving box's acceleration while this force is still being applied is: A) 5.0 m/s2 B) 2.0 m/s2 C) 0.2 m/s2 D) 0.5 m/s2