Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: The first ionization enthalpy of $3d$ series elements is more than that of group $2$ metals.
Reason $(R)$: In $3d$ series of elements,successive filling of $d$-orbitals takes place.
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 false but $(R)$ is true.
• D
  $(A)$ is true but $(R)$ is false.