Two point charges of $+10 \mu c$ and $-10 \mu c$ are placed at a distance $\mathbf{4 0} \mathbf{c m}$ in air. Potential energy of the system will be (a) $2.25 \mathrm{~J}$ (b) $2.35 \mathrm{~J}$ (c) $-2.25 \mathrm{~J}$ (d) $-2.35 \mathrm{~J}$