Assertion $(A)$: The first ionisation energy of $Be$ is greater than that of $B$.
Reason $(R)$: $2p$ orbital has lower energy than $2s$ orbital.

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