Answer :
The closing of the frontier came because D. Fast and affordable travel across the continent encouraged western settlement.
The Frontier referred to the portion of the West that had not yet been properly settled by the Americans.
It was considered dangerous because:
- there were frequent Native American attacks
- a low volume of settlements meant you had to travel long distances to get help
- outlaws and gangs roamed the land
The frontier was eventually closed when Americans were able to reach and settle in all areas of the United States because travel became cheaper and faster. For instance, the transcontinental railroad allowed people to get to the west in mere days.
In conclusion, better transportation enabled the settling of the west such that the frontier was closed.
Find out more at https://brainly.com/question/20194844.