D seems the most likely because ammonia is not as natural a substance as the others
With the cotton grin, cotton became much more worthwhile crop because ratio output quantity decrease a lot more. also the plantation owner can make a more money or a larger profit on the use of slavery workers.
D) To fight the spread of communism.
Because of the Domino Theory they thought that if Vietnam fell into Communism, the whole region would follow. So therefore they wanted to support South Vietnam in order to contain Communism in Asia.
The answer would be the second option. The price of gasoline had gone done prior to this shift.
