Four point charges of magnitudes $+ q$, $+ q$, $- q$, and $- q$ are held in place at the corners of a square of side $r$.

The Coulomb constant is $k$. Which of the following is the electrical potential at the centre of the square O?

A.     0

B.     $\frac{{4kq}}{r}$

C.     $\frac{{4kq\sqrt 2 }}{r}$

D.     $\frac{{ - 4kq\sqrt 2 }}{{{r^2}}}$

A