Each of the Four Corners of a Square with Edge

Multiple Choice

Each of the four corners of a square with edge a is occupied by a point mass m.There is a fifth mass, also m, at the center of the square.To remove the mass from the center to a point far away the work that must be done by an external agent is given by:

A) 4Gm²/a
B) -4Gm²/a
C)
D)
E) 4Gm²/a²