Sectional differences were increased: a. as the northeast developed into a strong industrial center b. when the invention of the cotton gin made cotton more profitable c. when pioneers moved west and demanded internal improvements d. all of the above e. none of the above

The answer is:  a. as the Northeast developed into a strong industrial center

Sectional differences refers to both cultural and economic difference that happen between american states after the civil war.  

Not long after the civil war, the northern states keep developing a strong industrialization in their territories while the southern states were left in disarray since the majority of their free labors have become a free citizen. This cause the northern states to develop faster economic growth than the southern states. and created the sectional difference.

answer; /// i do believe that the correct answer is (d) majority vote as an approval requirement;