Two thin conducting concentric shells of radii $R$ and $2R$ are shown in the figure. The outer shell carries a charge $+Q$ and the inner shell is neutral. When the switch $K$ is closed,which of the following statement$(s)$ is/are correct?
$(a)$ The potential on the inner shell becomes zero.
$(b)$ The charge on the inner shell is $\frac{Q}{2}$.

• A
  $(a)$ and $(b)$ are correct
• B
  $(a)$ is correct,$(b)$ is wrong
• C
  $(a)$ is wrong,$(b)$ is correct
• D
  $(a)$ and $(b)$ are wrong