Consider two Carnot engines C1 and C2 operating in series, such that the heat discarded by C1 is used as the heat input for the C2 cycle. If the intermediate temperature is the geometric mean of the two extreme temperatures, which option correctly describes the relationship between their thermal efficiencies?

Statement: An ideal black body is a perfect absorber and perfect emitter of thermal radiation. Conclusion I: Thermal radiation incident on a black body is neither reflected nor transmitted. Conclusion II: A black body emits more total energy per unit area at higher temperatures. Choose the correct option:

Which equation correctly represents the Stefan-Boltzmann law for blackbody radiation?