Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Efficiency of a reversible heat engine will be highest at $-273^{\circ} C$ temperature of cold reservoir.
Reason $R$: The efficiency of Carnot's engine depends not only on the temperature of the cold reservoir but it depends on the temperature of the hot reservoir too and is given as $\eta = (1 - \frac{T_2}{T_1})$.
In the light of the above statements,choose the correct answer from the options given below:

• A
  $A$ is true but $R$ is false
• 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
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$