$A$ parallel plate capacitor of capacitance $C$ is connected to a battery and charged to a potential difference $V$. Another capacitor of capacitance $3C$ is similarly charged to a potential difference $3V$. The charging battery is then disconnected and the capacitors are connected in parallel to each other in such a way that the positive terminal of one is connected to the negative terminal of the other. The final energy of the configuration is

• A
  $\frac{3}{2} CV^2$
• B
  $8 CV^2$
• C
  $\frac{13}{2} CV^2$
• D
  $18 CV^2$