The work required to bring a unit positive charge from infinity to a mid-point between two charges $20 \mu \mathrm{C}$ and $10 \mu \mathrm{C}$ separated by a distance of $50 \mathrm{~m}$, is :
(a) $10.8 \times 10^{4} \mathrm{~J}$
(b) $10.8 \times 10^{3} \mathrm{~J}$
(c) $1.08 \times 10^{6} \mathrm{~J}$
(d) $0.54 \times 10^{5} \mathrm{~J}$