The expansion of slavery into newly acquired territories was the main reason for the development of the American economy. So the correct option is D.
The development of the American economy has several factors which give the boom in the economy such as capitalization of the economy, green revolution boosts the agriculture sector.
The workers used to work more than usual time it cause the overproduction and it increases the real income of the people. There are abundant resources of energy in North America that makes the American economy more reliable and independent.
Therefore the correct option is D.
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