Which piece of evidence best explains how cotton and tobacco led to the expansion of slavery in the south? a while new england's development centered on trade, and the middle colonies fed america, the southern colonies turned to cash crops. b virginia was the first successful southern colony. immediately to its north was maryland. in both colonies, tobacco became the most important crop. c the south's focus on growing cash crops led to the development of a slave labor force that was largely unknown north of maryland. d slaves were present in the north but were much more important to the south. they were the backbone of the southern economy.