Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Barium carbonate is insoluble in water and is highly stable.
Reason $R$: The thermal stability of the carbonates increases with increasing cationic size.
Choose the most appropriate answer from the options given below for $A$ and $R$.

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