The right answer is The railroads and improved communication made expansion easier.
The Second Industrial Revolution began in the mid-nineteenth century and was centered in the United States and Germany. It was spurred by three related developments. The first was the creation of interconnected transportation and communication networks, which facilitated the emergence of a national and even international market for American goods and services. Contributing to this development was the completion of the national telegraph and railroad networks; the emergence of steamships, which were much larger and faster than sailing ships; and the laying of the undersea telegraph cable, which spanned the Atlantic Ocean and connected the United States with Europe.