"On August 10, 1821, Missouri entered the Union as the 24th state. Named after the Native American people who originally inhabited the land, Missouri was acquired by the U. S. as part of the 1803 Louisiana Purchase. At the time, the territory's occupants were mainly French settlers. After the war of 1812, American settlers poured into the region.

In 1818, the speaker of the House of Representatives present the first petition of the Territory of Missouri requesting statehood. The question of Missouri's admission as a slave or a free state-led statesman Henry Clay to devise the Missouri Compromise of 1820, admitting Missouri as a slave state while admitting Maine as a free state, and prohibiting slavery in Louisiana Territory north of the parallel 36 30, Missouri's southern border." - Library of Congress

How would The simultaneous admission of Missouri and Maine help to maintain peace in the United States?
A- it defined the rights of slave and free states.
B- it expanded the geographical size of the nation.
C- it kept the number of slave and free states even.
D- it would promote popular sovereignty in the West.