31. Box A of mass m sits on the floor of an elevator, with box B of mass 2 m on top of it, as shown in the figure above. The elevator is moving upward and slowing down. F_A is the magnitude of the force exerted on box A by box B, F_B is the magnitude of the force exerted on box B by box A, and F_g is the magnitude of the gravitational force exerted on box B. Which of the following ranks the forces in order of increasing magnitude?
(A) F_B=F_A=F_g
(B) (F_B=F_A) (C) F_B<(F_A=F_g)
(D) F_g