4. A power cycle receives QH by heat transfer from a hot reservoir at TH = 1200 K and rejects energy Qc by heat transfer to a cold reservoir at Tc = 400 K. For each of the following cases, determine whether the cycle operates reversibly, operates irreversibly, or is impossible. a) QH = 900 kJ, Wcycle = 450 kJ b) QH900 kJ, Qc = 300 kJ c) Wcycle = 600 kJ, Qc = 400 kJ d) n = 75%