What Did President Lincoln Mean When He Said in His

What did President Lincoln mean when he said in his first inaugural address,"Physically speaking,we cannot separate"?

A) The costs of creating separate international borders between North and South were too high.
B) The North and South were bound together by geography without an obvious division by a mountain range or river.
C) The North and South were bound tightly together by family ties.
D) The North and South had to remain united because they were sandwiched between the great powers of Canada and Mexico.
E) The railroads connecting North and South had created too many commercial and transport ties for the regions to separate.