Railroads connected vast regions of the us and increased efficiency for the transport of goods. the expansion of railroads welas made possible because of. a. the oil industry b. the telephone c. the steel industry d. the iron industry

The expansion of railroads was made possible because of the Steel industry.

When the tremendous growth of Steel industry took place in 19th century, as a result of a strong technological foundation, steel became a crucial element for rapid expansion of urban infrastructure; which also closely included the expansion of railroads. Due to steel highways, the lives of millions of people were improved through the use of railroads which supplied the expanding U.S. cities with food, fuel, building materials and access to markets.  

d.)   when the us left nato after the end of the cold war and they had to evolve to focus on more humanitarian measures with the smaller budget