Two Boxes, a and B , Are

Multiple Choice

Two boxes, A and B , are connected by a horizontal string S on a horizontal floor. A very light wire pulls horizontally on box B, as shown in the figure, with a force of 100 N. The reaction force to this pull is

A) the pull that string S exerts on box A .
B) the force that box A exerts on box B .
C) the pull that box B exerts on string S.
D) the pull of box B on the wire.
E) the pull that string S exerts on box B.