Well, the chinese big secret was silk. So that would be my best guess.
The correct answer is D) "Population in rural areas grew faster than urban areas".
After the <em>Treaty of Guadalupe Hidalgo</em>, where Mexico ultimately gave the US control of Texas in exchange for an economic compensation, the border between both countries was established at the Rio Grande.
This finally paved the way for immigrants to come to Texas and settle in the lands where cotton production was possible, resulting in a rapid growth of rural populations.
The US had many attempts at compromise with varying levels of sucess before the Civil War. However, when Lincoln was elected president (on the platform of stopping the expansion of slavery), the South lost hope and seceded.
There is always a solution other than war, however, in this case, war had been a looming possibility for quite some time, and compromise probably would've only pushed the war back a year or two.
Hope this helps :)
