In the circuit shown in the figure, there are two parallel plate capacitors each of capacitance $C$. The switch $S _1$ is pressed first to fully charge the capacitor $C_1$ and then released. The switch $S_2$ is then pressed to charge the capacitor $C _2$. After some time, $S _2$ is released and then $S _3$ is pressed. After some time.
$(A)$ the charge on the upper plate of $C _1$ is $2 CV _0$
$(B)$ the charge on the upper plate of $C _1$ is $CV _0$
$(C)$ the charge on the upper plate of $C _2$ is $0$
$(D)$ the charge on the upper plate of $C _2$ is $- CV _0$
