Final answer:
The building of a network of roads, an increase in travel and trade, and the spread of a common language in ancient Rome were primarily caused by the gradual unification of the Italian peninsula under Roman rule.
Step-by-step explanation:
The building of a network of roads, an increase in travel and trade, and the spread of a common language in ancient Rome were primarily caused by the gradual unification of the Italian peninsula under Roman rule. As the Romans expanded their empire, they built cities with efficient infrastructure, including grids of streets centered around public forums, which facilitated trade and travel. These cities were connected by a vast network of roads, totaling 40,000 miles, enabling efficient communication and transportation. Additionally, the Romans spread their language, Latin, throughout their empire, which eventually evolved into the Romance languages.