A two-stage heat engine works as follows:
Stage 1 absorbs Q1=4000J from a reservoir at 600 K, performs W1=1000J of work, and rejects Qx to a reservoir at 400 K.
Stage 2 uses this rejected heat Qx as its input, performs W2=2500J of work, and rejects 500 J to a reservoir at 300 K.
Which conclusion about the overall device is correct?
(a) It is permissible because energy is conserved and some heat is rejected.
(b) It violates the Kelvin–Planck statement since it converts heat from a single reservoir entirely into work.
(c) It violates the Kelvin–Planck statement because its overall efficiency exceeds the maximum possible between 600\,\text{K} and 300\,\text{K}.
(d) It violates the First Law because the total work output exceeds the net heat absorbed.