In which pair of events did the first event most directly influence the second?

Adiscovery of gold in California — Louisiana Purchase
Bbuilding of the transcontinental railroad — disappearance of the frontier
Csettling of the Oregon Territory — passage of the Homestead Act
Dassimilation of Native American Indians into American Society — passage of the Dawes Act

Verified Answer :

The correct answer for the question that is being presented above is this one: "B. building of the transcontinental railroad --> disappearance of the frontier." The pair of events that did the first event most directly influence the second is the building of the transcontinental railroad --> disappearance of the frontier.