All Real Engines Are Less Efficient Than the Carnot Engine

Multiple Choice

All real engines are less efficient than the Carnot engine because

A) they do not operate through the Otto cycle.
B) they do not operate through a reversible cycle.
C) the working substance does not maintain a constant volume through the cycle.
D) the working substance does not maintain a constant pressure through the cycle.
E) the working substance does not maintain a constant temperature through the cycle.