Why does the margin of error decrease as the sample size n increases?

Which of these describes a social effect of the civil war? a) fields were left unplanted because the slaves had been freed and much of the male population was unavailable to plant or harvest crops. b) over 600,000 men died, mostly because of the lack of food, clean water, and hygienic medical practices. c) after the emancipation proclamation was issued, african americans were allowed to join the union army. d) lincoln’s assassination after the surrender caused a disruption in the rebuilding of the nation.