In the first paragraph, the author implies that writers' reactions to the development of railroads were ______.

A．They have as much literary importance as the books written by Emerson, Thoreau, and Adams.
B．They are good examples of the effects industry and business had on the literature of the United States.
C．They contributed to the weakening of traditional values.
D．They are worth reading as sources of knowledge about the impact of railroads on life in the United States.

A．many of its writers had experience working on railroads.
B．many of the books were set in railroad stations and yards.
C．the books were well known during the railroads' prime years.
D．quite a few of the books are still popular today.