A student notes that a real heat engine always delivers less work per joule of heat absorbed than an ideal reversible engine working between the same two reservoirs. Which of the following statements best explains this observation in terms of entropy?
(a) Because entropy must always *decrease* in spontaneous processes, the real engine loses some usable energy as its entropy falls, giving less work.
(b) The real engine operates at a lower average temperature, so the amount of energy it receives for each joule of heat is smaller than in a reversible engine.
(c) The extra entropy generated inside the real engine forces additional energy to be expelled as waste heat, leaving less that can be converted to work.
(d) Irreversibilities convert part of the heat input into additional internal energy that remains trapped in the working fluid instead of being extracted as work.