Two Boxes,A and B,are Connected by a Horizontal String S

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