Given below are two statements: one is labelled as "Assertion $A$" and the other is labelled as "Reason $R$".
Assertion $A$: In the complexes $Ni(CO)_4$ and $Fe(CO)_5$,the metals have zero oxidation state.
Reason $R$: Low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.
In the light of the above statements,choose the most appropriate answer from the options given below.

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