Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Size of $Bk^{3+}$ ion is less than $Np^{3+}$ ion.
Reason $R$: The above is a consequence of the lanthanoid contraction.
In the light of the above statements,choose the correct answer from the options given below:

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