The answer is
B - It won the Americans to a foreign ally.
Commander in chief of the National Revolutionary Army (from which he came to be known as Generalissimo), he led the Northern Expedition from 1926 to 1928, before defeating a coalition of warlords and nominally reunifying China under a new Nationalist government.
True, the disease spread to the Aztecs greatly diminished their population, making them much easier to conquer.
Before WWI America owed Europe a lot of money, but after WWI the situation changed and the former allies owed America more than US$ 10 Billion for the cost of armaments and food supplies.
After the WWI a decade of tremendous prosperity started. This economic boom was a consequence of the mass production industries increasing gross domestic products, productivity and income, so business sales increase, driving up profits.
