A Carnot engine operates between temperatures T1 and T2 (T1 > T2). If the high-temperature reservoir is cooled from T1 to T1/2 while keeping the low-temperature reservoir at T2, what is the new efficiency?
Correct: D
Efficiency of a Carnot engine is η = 1 - T2/T1. After cooling the high-temperature reservoir to T1/2, the new efficiency becomes η' = 1 - T2/(T1/2) = 1 - 2T2/T1. Thus, the correct answer is D.