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.
It is B i just did that test and i alredyknow
<span>Spain's conquest of central Mexico and the Andes made Spain the wealthiest and most powerful nation because of the abundance of gold and silver which it gained. This large amount of gold and silver it was able to obtain was sent to its national treasuries allowing it to become a threat to any other European competition.</span>