Joseph stalin demonstrated the main advantage of a command system by
a.
increasing industrial output.
c.
introducing the new economic policy.
b.
eliminating shortages.
d.
increasing the standard of living.

I would say the answer is A. increasing industrial output because Stalin did do that and the definition of a command economy is "an economy managed by the government, in which industry is publicly owned." Under Stalin's rule, the production of industrial products like steel greatly increased.