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 $–273 °\mathrm{C}$ temperature of cold reservoir.

Reason R : The efficiency of Carnot’s engine depends not only on temperature of cold reservoir but it depends on the temperature of hot reservoir too and is given as $\eta =\left(1-\frac{T_2}{T_1}\right)$

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
• Correct answer
  D

  Both A and R are true and R is the correct explanation of A

Hence, the correct answer is option: 'D'.