What are large particles that are rolled or pushed on the bed of the stream called?

Which of the following would not be considered a geographic impact on the history of middle and south america? a. the relative geographic isolation of the americas meant that they would be more susceptible to european diseases. b. ernesto “che” guevara's trip throughout south america inspired him to become a revolutionary. c. the tropical climate of north america has encouraged many south american countries to produce goods there. d. the proximity of the united states to middle and south america has strengthened its relationship to the region.