In each of five situations, two point charges $\left(Q_{1}, Q_{2}\right)$ are separated by a distance $d .$ Rank them in order of the electric potential energy, from highest to lowest.
(a) $Q_{1}=1 \mu C, Q_{2}=2 \mu C, d=1 \mathrm{m}$
(b) $Q_{1}=2 \mu \mathrm{C}, Q_{2}=-1 \mu \mathrm{C}, d=1 \mathrm{m}$
(c) $Q_{1}=2 \mu C, Q_{2}=-4 \mu C, d=2 m$
(d) $Q_{1}=-2 \mu \mathrm{C}, Q_{2}=-2 \mu \mathrm{C}, d=2 \mathrm{m}$
(e) $Q_{1}=4 \mu \mathrm{C}, Q_{2}=-2 \mu \mathrm{C}, d=4 \mathrm{m}$