Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A:$ The first ionisation enthalpy decreases across a period.
Reason $R:$ The increasing nuclear charge outweighs the shielding across the period.
In the light of the above statements,choose the most appropriate from the options given below:

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