Carnot's result implies that a heat engine could reach 100% efficiency only under one specific condition on the cold reservoir. What is that condition, according to the source?

1. The Carnot efficiency formula is EffC = 1 - Tc/Th, so EffC equals 1, or 100%, only when the fraction Tc/Th equals zero. 2. Since Th is never zero for a real hot reservoir, that fraction can only vanish if Tc itself equals zero, meaning the cold reservoir sits at absolute zero. 3. Reaching absolute zero for a real reservoir is called a practical and theoretical impossibility, which is exactly why no real heat engine reaches 100% efficiency. 4. Making the two reservoirs equal in temperature, as in option A, would instead drive the efficiency to zero, not to 100%, since there would be no temperature difference to exploit. 5. Room temperature and a doubled cold-reservoir temperature, options B and D, do not correspond to the zero-efficiency-loss condition the formula requires. _Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.4 Carnot's Perfect Heat Engine: The Second Law of Thermodynamics Restated_