The map on the left shows the railroads that were built in the United States before the end of the Civil War. The map on the right is a topographic map of the continental U. S. showing the country’s major geographic features. What geographic feature seems to prevent the growth of railroads?

  Appalachian Mountains

Explanation:

Few rail lines cross the Appalachian Mountains, and the various north-south ranges of mountains in Pennsylvania effectively prohibit the construction of east-west rail lines in that area. The same holds for northern Georgia and Alabama.

It is interesting that the Ohio River valley also doesn't have much growth of railroads, perhaps because river transportation effectively competes with rail transportation in that area.

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the answer is d. selective incorporation. (apex)