A and C can work and D is partially true I would go with a or d tho
Answer: the invention of cotton gin
Explanation:The cotton gin refers to a a machine that sets aside cotton fibers from their seeds. Therefore, the invention of the cotton gin transformed the cotton industry in the United States and demanded cheap labor force, which led to the rise of slavery in the South and to the American Civil War later.
It was primarily "<span>d. the vulnerability to invaders from the north", since the Mongol territory was rapidly expanding--making the Mongol warriors all the more powerful. </span>
