How Was Korea Separated Between North and South Korea

How was Korea separated between North and South Korea?

A) South Koreans had always been mistreated by the dictators in North Korea, so they waged a Civil War against them and got their independence.
B) North Koreans are actually closely related to the Chinese, while South Koreans are "true" Koreans. They decided to split based on ethnic backgrounds.
C) It came as a result of the United States and Russia trying to figure out how to share it after World War II.
D) A terrorist group in North Korea assassinated the leader and began a war that was not won decisively by either side.