The Haitian Revolution has often been described as the largest and most successful slave rebellion in the Western Hemisphere. Slaves initiated the rebellion in 1791 and by 1803 they had succeeded in ending not just slavery but French control over the colony.
Because maybe bad things will happen or if there is a war
The Korean War was the second war in which the United Nations played a military role. The correct option among all the options given in the question is the second option. It was the toughest test for the United Nations after its inception. It was clear that the United Nations forces sent to Korea was dominated by American soldiers, but several other countries did participate in it.
The growing of corn enabled the native Americans to have a much more reliable food source that was produced in large quantities and they also had reserves for bad times. This allowed the populations to grow significantly, and also larger cities to be built and developed. It also contributed that the people in this part of the world to have more time to focus on other things, like sciences, astronomy, architecture, instead of just living day by day and be in a constant search of food.