Final answer:
The two statements that best summarize the text are b) innovations in transportation and westward expansion in the United States and d) how transportation and communication unified U.S. citizens.
Step-by-step explanation:
From the provided text and options, the two statements that best summarize the main ideas of the text are:
b. Historically, innovations in transportation and westward expansion in the United States.
d. Transportation and communication have served to unify United States citizens.
The texts discuss the rapid growth of the United States in the late 1800s and how industrialization and urbanization were propelled by advancements in technology and infrastructure, such as railroads and the telegraph. As the United States expanded westward, innovations in transportation and communication played critical roles in unifying the nation and fostering urban growth, while contributing to the migration patterns within the country.