Final answer:
The growth of southern cotton into a leading commodity was guided by the combination of the rise of cotton in the 1800s and the invention of the steam engine.
Step-by-step explanation:
The rise of cotton in the 1800s combined with the invention of the steam engine guided the growth of southern cotton into one of the world's leading commodities. The steam engine revolutionized transportation, making it faster and more efficient to transport cotton from plantations to textile mills. This led to a significant increase in the production and export of cotton in the southern United States.
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