Assertion $(A)$: $K, Rb$ and $Cs$ form superoxides.
Reason $(R)$: The stability of the superoxides increases from $K$ to $Cs$ due to decrease in lattice energy.
The correct answer is

• 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 true but $(R)$ is not true
• D
  $(A)$ is not true but $(R)$ is true