Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$ : The stability order of $+1$ oxidation state of $Ga$,$In$ and $Tl$ is $Ga < In < Tl$.
Reason $R$ : The inert pair effect stabilizes the lower oxidation state down the group.
In the light of the above statements,choose the correct answer 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
  Both $A$ and $R$ are true but $R$ is $NOT$ the correct explanation of $A$.
• D
  $A$ is false but $R$ is true.