A cyclic engine using an ideal gas executes the following four successive steps:
I. Isothermal expansion at 400K from state A to B.
II. Adiabatic expansion from B to C.
III. Free expansion into vacuum from C to D until the gas pressure halves.
IV. Isothermal compression at 300K from D back to A.
Which single step must necessarily be modified or eliminated to make the entire cycle reversible?
a) Step II only
b) Step III only
c) Step I only
d) Step IV only