Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Phenolphthalein is a $pH$ dependent indicator,remains colourless in acidic solution and gives pink colour in basic medium.
Reason $R$: Phenolphthalein is a weak acid. It doesn't dissociate in basic medium.
In the light of the above statements,choose the most appropriate answer from the options given below.

• 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 false but $R$ is true.
• D
  $A$ is true but $R$ is false.