Four +600-µC Point Charges Are at the Corners of a Square

Four +6.00-µC point charges are at the corners of a square 2.00 m on each side.What is the electric potential of these charges,relative to infinity,at the center of this square?

A) 76.4 kV
B) 38.2 kV
C) 306 kV
D) 153 kV
E) 61.0 kV