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Read the passage from History of the United States, by Charles and Mary Beard.

As necessary to mechanical industry as steel and steam power was the great market, spread over a wide and diversified area and knit together by efficient means of transportation. This service was supplied to industry by the steamship, which began its career on the Hudson in 1807; by the canals, of which the Erie, opened in 1825, was the most noteworthy; and by the railways, which came into practical operation about 1830.

The railways followed the same paths [as the canals]. By 1860, New York had rail connections with Chicago and St. Louis—one of the routes running through the Hudson and Mohawk valleys and along the Great Lakes, the other through Philadelphia and Pennsylvania and across the rich wheat fields of Ohio, Indiana, and Illinois. Baltimore, not to be outdone by her two rivals, reached out over the mountains for the Western trade and in 1857 had trains running into St. Louis.

In a summary of this passage, which of the following is a central trend?

a. steel
b. industry’s need for a market
c. the Erie Canal system
d. the railways