Which object is most efficient?
a. A heat engine that produces 30 J of work for 60 J of input energy.
b. A heat engine that produces 15 J of work for 30 J of input energy.
c. A heat engine that produces 20 J of work for 60 J of input energy.
d. A heat engine that produces 40 J of work for 50 J of input energy.
