Electric Potential Energy Practice Test

A charge of $$+q$$ and a charge of $$-2q$$ are located on the x-axis at $$x=0$$ and $$x=d$$, respectively. How much work must an external agent do to bring a third charge of $$+3q$$ from infinity to the point $$x=2d$$ on the x-axis?

System A consists of two protons separated by a distance $$d$$. System B consists of a proton and an electron separated by a distance $$d$$. System C consists of two electrons separated by a distance $$d$$. Let $$U_A$$, $$U_B$$, and $$U_C$$ be the electric potential energies. Which option correctly ranks the potential energies?