First, they made settlement of the West possible after the Civil War by connecting farmers (and miners, ranchers, loggers, and others) to markets.
Second, railroads helped to integrate different regions of the country economically.
i believe the best answer of all the choices is d. hope it helps ask if you have any more questions.
Answer:
Primary . Examples include interview transcripts, statistical data, and works of art
Secondary Examples include journal articles, reviews, and academic books.
Explanation:
a model of the Solar System in which the Earth and planets revolved around the Sun,
