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.
This is a example of getting distracted
C. Produce commodities that Brazil needs and trade them for coffee.
The question explains that nations with different climates cannot make their own quality coffee, elimating D. Since they want COFFEE, that elimates B. Since Brazil can make their own coffee, why would they go to Brazil to grow it themselves? Its not logical. That elimates A, leaving C as the answer
C=> Undecomposed organic matter