13. A boy pulls a wooden box along a rough horizontal floor at constant speed by means of a force as shown. In the diagram f is the magnitude of the force of friction, N is the magnitude of the normal force, and Fg is the magnitude of the force of gravity. Which of the following must be true? (A) P = f and N = Fg (B) P = f and N ＞ Fg (C) P ＞ f and N ＜ Fg (D) P ＞ f and N = Fg (E) None of the above.