Final answer:
Railroads were more expensive to construct than canals but offered faster transportation, promoting economic growth and national unity. They required political intervention for standardization and coordination. Therefore, the correct option is A.
Step-by-step explanation:
Compared with canals, railroads were initially more expensive to construct, but they transformed commerce by offering rapid and reliable transportation across the United States. While canals provided cheaper shipping costs, railroads decreased travel time significantly and linked various parts of the country, promoting economic growth and the creation of a national market system. The construction of railroads required substantial investment and coordination, leading to increased political influence and the need for government oversight to standardize safety measures and ensure consistency in operations across different regions.