a) Incorrect. The economic system in the North evolved more rapidly towards industry and commerce. Slavery was not necessary in this new context.
b) Correct. Agriculture in large cotton or tobacco plantations required slave labor in order to develop.
c) Incorrect. It is not a precise answer because North America has border territories both in the north and in the south. On both sides the land policies were different.
d) The Great Plains is a vast territory that extends into ten states. Although in all of them prevailed systems of extensive agriculture, not all were prone to slavery labor.
The southern states were heavily dependant on the agriculture industry. They greatly benefited from the "Cotton Boom" during the first half of the 19th century, which made the United States one of the leading cotton producers in the world.
The agriculture plantation system used slaves as the primary workforce. They were seen as an exchangeable resource. As a result, a whole economic activity developed based on the slave trade.
The system ultimately declined with the abolition of slavery.
In the late 19th century, many US businesses had interests in Cuba, and part of the impetus for the Spanish-American War was to prevent its colonial government from adversely regulating trade with the US.
So C. to protect United States business interests in Cuba
