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:

$1. \frac{2}{1}$
$2. \frac{1}{2}$
$3. 1$
$4. \frac{2}{3}$