Four point charges are individually brought from infinity and placed at the corners of a square as shown in the figure. Each charge has the identical value +Q. The length of the diagonal of the square is 2a. What is the electric potential at P, the center of the square?

A) kQ/a                           B) 2kQ/a                        C) 4kQ/a                        D) kQ/4a                        E) zero volts