The main reason why the border states did not secede in 1860-1862 was because they were concerned with the large amount of military coercion that was taking place in the South, and they felt that the South was ultimately unjustified in removing itself from the Union.
The answer is C, and the reason why is because the climate changed and since they where not used to that, that affected them, also rainfall and flooding started to shrink and that also contributed to the downfall of the Old Kingdom.