A 900 pF capacitor is charged by a 100 V battery. The electrostatic energy is stored by the capacitor is U. If the capacitor is disconnected from the battery and connected to another 900 pF capacitor (as shown in the figure ) the electrostatic energy stored by the system is U'. The ratio of $\frac{U}{U\text{'}}$ is:

$1.<mspace\; width="1em"></mspace>\frac{2}{1}$
$2.<mspace\; width="1em"></mspace>\frac{1}{2}$
$3.<mspace\; width="1em"></mspace>1$
$4.<mspace\; width="1em"></mspace>\frac{2}{3}$