Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: The ionic radii of $O^{2-}$ and $Mg^{2+}$ are same.
Reason $R$: Both $O^{2-}$ and $Mg^{2+}$ are isoelectronic species.
In the light of the above statements,choose the correct answer from the options given below:

• A
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
• B
  Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$
• C
  $A$ is true but $R$ is false
• D
  $A$ is false but $R$ is true