Why does it go back 1 day from west to east through the meridian of 180 degrees? and why does it add 1 day from east to west across the meridian of 180 degrees?

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.